No, i never kept card what I wanted to send…
When I buy postcards, I often buy postcards for myself. Sometimes, when I really like the motive I buy the postcard up to three times. One for me and the others to send away. ^-^
It happened once that I bought a postcard many, many, many years ago and then I didn’t like this motive anymore. Can’t remember which motive it was, but I send the postcard away.
Postcards with tea themes I always keep. Even if I dislike the motive, but as a tea addict I want to have as much as possible tea themed postcards.
Now and then I am buying postcards just for myself. I used to do this even before postcrossing, maybe once a year or every two years when the mood did strike I carefully chose a couple of cards. I have a magnetic surface in the kitchen where I hang 3 or 4 favourites and rotate when I feel like it (every few months). Sometimes I also buy postcards during travels for myself as a souvenir. Personally I don’t think of these as postcards for posting though, they are more in the category “home decoration” 
I have a story about a failed “Oh that’s beautiful, but I’m buying it just for sending” incident though 
A few weeks ago I bought a postcard I really really liked with the full intention of sending it through Postcrossing. I really did so, but after sending it I had to think about the motive so much, that I finally went out and hunted down two more. One just for myself to keep, and one to send off in case the first one doesn’t get registered, because I really wanted to have that picture on my wall too 
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