Whenever I'm ready to sell loads of cards after hours of grinding, I will go "work offline" or turn off my modem. After selling those cards, I will do a quick check to see if there's any card that was accidentally sold. If no, I'll reconnect and let it save.
Just a small tips; provided that you have a good memory or some sort of spreadsheet for your card collection.
Btw, I'm pretty sure only mods can move this topic. Don't worry it'll be move shortly.