If they don't respond in a few hours, it's probably because they're not online. Just wait and give them time a bit, and if they don't respond for weeks and weeks, and you know that they were online the past couple of weeks and just plain ignoring you, then I think it's safe to say that you can give the auction item/items to someone else. I've had this happen once when I was holding a giveaway, and that guy who won a Gracie Set just plain ignored me, and he wasn't even new to TBT, so I knew it wasn't because he didn't know how to reply back...sorry for the long message, just ranting.