I don’t do private swaps unless it’s here on the forum, I think I’d ignore messages about swaps that come to me any other way. But that hasn’t happened yet.
I never send a card back to an official Postcrossing sender. I understand that if you get to send to a rare country that it would be cool to get one back, but that’s not how Postcrossing works. I’m sorry you have so many requests, but I wouldn’t feel bad about not sending anything to be honest.