Maybe you should add more auctions with different times, some of them like only 1 hour or less.
As of now auction time is 6 hours, i.e. there are 4 auctions per day: this should cover all time zones. With more user engagement yes, we could do more than 4 per day.
If I send an transaction in the last 5 minutes, and it hasn't confirmed, but someone else outbids me, I can double spend that transaction, giving me an (almost) risk free chance of winning, and possibly cheating others to force them to make higher bets. Consider adding a policy that the transaction only counts as a valid bid if it confirms before the end time. Will combat sniping, and will also make the game more interesting.
Interesting... without confirmation you shouldn't be able to get the prize anyway, but I see your point. Maybe requiring 1 confirmation before auction end in order for the bid to be counted could be an interesting twist.
Thank you all guys, your feedback is very much appreciated!