I don't see a problem with bidding on auctions when you're going to be away. On the other hand, I'd suggest timing your auctions to end when you're going to be away for more than a couple days isn't being a responsible seller. Note, I'm not suggesting that anyone has an obligation to answer unsolicited emails promptly. But my feeling is if you are selling something in an online auction, the responsible seller will be around to answer questions and to contact the successful bidder.