In the same way you have to be ready to accept that cards you give people can end up in the trash...once you sell it, you need not to worry about it. We have no way to control how people mail cards they buy from us.
What I do is sometimes I include a very thin piece of foam like I saw in commercial cards which you can buy by sheets in packing material sections that I cut down to A2 size. I might also have a note on the clear envelope it's in to address the envelope before putting the card in.