In such cases as Hrghorhg's post, it's considered a valid and binding bid. However, including the question mark makes it a bit misleading for other potential bidders and comes close to disallowed practices outlined in the Auction Rules for Bidders, so I'll remind all Auction thread readers to please limit posts to serious bids and relevant questions/comments only. Thank you ^_~
I think the question mark implied he didn't know whether the auction has ended or not, so it was just a regular bid, but he wasn't exactly sure if it was still valid or not. Also, there was still about 4.5 hours left before the auction would've ended, so this auction continues!