I have had some success with a block-chain auction.  That might work for you.

In it's simplest form, you post and address and announce what block the auction will end.
Bidders pay their bid to that address.
You may want to extend the auction 10 more blocks from the payment block if a bid comes in before the ending block.
And of course, return the non winning bids.

TheJuice has written a web page to help follow the totals in such auctions.