I think a lot of people wouldn’t like that. I had that thought in my early days but as soon as I started hanging out on the forum I realised it’s quite a big no-no, or at least that’s how it sounded. I’m surprised several of the posts above would be okay with that!
(I didn’t know until now that there was a tag for this type of cards!)
I think you mean this one:
I don’t like cut out greeting cards, but I do like hand made cards a lot 
(for me these are very different thing - the other some one made, and the other is just a part of something). But of course some only like certain type of card (no hand made), and I understand them too.
I wouldn’t send these, unless someone writes they like these. Maybe join some greeting card swaps until you find postcards, @anon87497032?
Also I think of the waste of the other part of greeting card. If a greeting card is lovely, I like to keep them standing somewhere, so it feels not nice to cut them.
I think it’s okay as long as you cut them carefully. I sent two greetings cards shaped into postcards when I first joined Postcrossing a few months ago (back when I wasn’t a member of this forum and wasn’t aware I was commiting a crime - jk 
). The recipients seemed to be okay with that, judging from the heartwarming hurray message.
However, I wouldn’t send a cut greeting card to someone that says they don’t like homemade postcards, so perhaps it’d be a good idea to order some postcards online, since it’s hard to find some near you.
Maybe this wiki with online shops sorted by country will help:
Thanks guys. I must say I am surprised, but I will not do it. I read someone had cut out a cereal box and sent it as a postcard so I thought a top quality nice greeting card front with images and motifs from New Zealand would be okay. I would keep the backs to do handmade cards so no waste 
Yes, some do so, but these are not liked by many either (the food pack cut out cards). But someone like them too.
Still, I only send these “modified” parts only to someone who mention liking them.
(Of course, my own experience affects a lot on how I think about these. Right now I am trying to solve an id of a card that seems to be exactly cut out of a used greeting card. Somewhat floppy, nothing written by hand, only printed address, printed id (part of torn out), printed message, also torn out. And I’m thinking whyyyyy in the world I’m spending time to get this registered, when I’m most obviously only part of a bulk sending line 
)
I speak only for myself, but I’m happy to get pretty much any postcard someone sends me, provided they took some amount of care with it. If it were (as others said) a repurposed greeting card sloppily cut with a scissor, I’d be unhappy not because it was in some odd way not a “real” postcard, but because the sender clearly didn’t much care about what they were sending or who was getting it.
On the topic of “real” postcards, I tend toward the minimalist definition which (checks) I think used to be here, but I guess is in one of the deleted posts. I’ll try to accommodate requests by recipients; if they really hate Zazzle cards (for example), it doesn’t matter if I don’t know why or disagree with them – I’ve got plenty of other cards I can send, and I’m doing this because I find joy in it and because I’d like to send a tiny parcel of happiness to someone else. Some of my nicest (to me) cards are a set I got from JetPens with images from “My Neighbor Totoro”. Beautifully (and of course professionally) printed, but would run afoul of some folks’ preferences because they’re slightly larger than the standard size. I have other very nice cards which are a bit smaller than standard, too.
I’ve received cards of various sizes, just yesterday got a square card from the Netherlands, and outside the auspices of Postcrossing I recently received a matched pair of cards (each with one half of a message) send on decorated pieces of cardboard. Every last one of them made me happy. I’ve been thinking about converting some board game boxes into postcards, though I’d likely only send something like that to a recipient who explicitly requested handmade cards (and who has board games as an interest).
I actually just read the FAQ about this:
Postcrossing is a postcard exchange community, so each exchange that you make must include at least one postcard. It can be a postcard you bought in a store or a handmade postcard, as long as it hasn’t been previously used.
Also just to add that I am happy with any postcard. Anything in my mail that isn’t a bill makes me happy. 
You need to be a bit artsy/accurate for it to be well done.
Things like cutting a straight line when you cut the back of.
And it must still be in a form so that mail services accept it.
I had someone in Finland who expressly requested “handmade”.
So I glued back and front of a blank letter card together to make it thicker,
then glued the label of a water bottle on that, also painted a small rabbit for him as a rabbit fan.
He was pleased enough with it.
I ALWAYS read the intros to send something that is likely to be to the person´s taste.
I merged 2 topics together.
Here are the quotes from the last posts written here before I added the other topic (although they are -Off Topic- 
):
Well, I think the worries and fears people have for cut greeting cards can actualize in “real” store-bought postcards too.
For example:
The front of a greeting card can be sloppy and a postcard sturdy… but it can also be the other way around.
The front of a greeting card does not have artist’s name or other information… but sadly, neither does every postcard either.
The front of a a greeting card is “free”, or “a cheater’s way to join”… but postcards can also be free or really cheap (like many popular postcard box cards).
The front of a greeting card has a boring picture… well, so do many postcards. Everybody has their own preferences.
The front of a greeting card is accompanied by printed text, printed postage stamps, unclear ID or something else… well, there is no protection against them with postcards either.
I wonder, if the receiver thought they had received a cut greeting card front, but instead got a legit postcard and were told so - would they feel different?
Yes, my ideal postcard matches my interests, is sturdy, has artist’s name and other information, perhaps even some specially designed backside decoration. But not all postcards are like that.
Summary:
My three recommendations for someone who cuts greeting cards: use only sturdy cards, cut them cleanly as possible, and try to find information about the subject/artist (Google image search might help - or perhaps something is printed on the backside), so you can tell the recipient about the card.
I like handmade, but to me these greeting card parts are not hand made, because they are machine made.
I’d only send them in a tag or to people who mention wanting such
(And also look that the message written in the card originally isn’t imprinted to other side too 
because I’ve received such. No matter how beautiful excuses people try to come up, the truth is, many people who send these, only tear up the part of the used greeting card they got, scribble something and send it away. (Which is fine in a reuse swap.))
Often people are too kind and don’t tell the card was a disappointment. (And I fully understand it.)
(I also understand that we must register these, unless it’s used or underpaid etc.)
I like this attitude, and this is what people often say.
But, there are people like me, whose most of mail is other than bills.
Then it’s nice and expected to get a postcard from a postcard swap, not just anything cut resembling a postcard shape.
I think there’s nothing wrong with cutting a greeting card into a postcard.
Just cut neatly. I have plenty of blank unused greeting cards and I’ve considered doing the same before.
Obviously because of their size, just use first class stamps instead of the postcard rate stamp.
I understand it can be an availability or money issue and you just have to use what you have, so it’s not a problem for me personally.
Like other people said, as long as you cut neatly, no one’s really gonna know. Maybe glue a thicker piece of card stock/cardboard because I know some cards can be thin. But then again, flimsy postcards exist, so 
Wow @Izzy2018 January 23rd is when the phrase postcard police was coined by someone else. Then @S_Tuulia said she might be one. She herself admitted to it. I agreed.
We need a touch more graciousness. I personally would be pleased to receive a card that had been made into a postcard. I would never tell someone that their card didn’t meet my expectations. Never. If someone dislikes my card and the thought and expense i put into it, i would rather not hear from them.
Graciousness Definition & Meaning | Dictionary.com
the quality or state of being benevolent, courteous, and kind
Yikes, you guys. 
I wasn’t referring to @S_Tuulia or anyone else in particular when I referred to the “postcard police” almost a year ago, and in fact, I appreciate her perspective. Please kindly settle this dispute elsewhere.
From my part there is no dispute.
I have apologised her twice over a quarter of a year ago (for not liking that my address is shared to a third party, I believe this is the thing). That is enough.
This way of dealing things (bringing it up here after so long time) is also something I don’t like.
If she chooses to forever play victim and use my “rude” message this way, it’s something she needs to deal with herself.
Or maybe this is the time for practising graciousness and compassion, towards me.
But, back to topic.
Why this “don’t tell and no one will know” attitude someone seem to have? Why not be straight and say, I cut this from this and that 
? If they think it’s perfectly ok, why then hide what they do?
I’d prefer they tell me this if they can’t send the whole greeting card.
And they can, because:
