When i trade/sell I always keep a note book (the real paper kind, nothing on the computer) and I write the person's name/address/email address/What we're trading (or I'm selling, eitherway pinpics numbers only)/delivery confirmation number and finally when it was delivered.
Now, with some of my pins I have more then one of them, so when writing down the pinpics numbers, if I have more than one, I don't do anything with it, if I only have the one, I make it a point of highlighting it with a marker. That way, I know when I go to ship the trade or after the person pays, I can update my notebook and remove the pin(s) that are highlighted. Not fool proof, but it works well enough for me and then I have a hard copy of anything I've done π