It is possible to create a small transaction output as an anchor for each auction: Interested parties would then be able to place their bids by signing a transaction that uses both their bid and the anchor as inputs.
When the time-frame of the auction ends, the auctioneer can countersign the transaction with the highest bid, and since the anchor can only be spent once, all other bids are automatically invalidated.
The transactions would have to be posted through the auction's portal, as they wouldn't be relayed by the network without being signed completely. After they were posted though, everybody could verify that they are valid and the bidder actually has the funds.
One problem is though, that bidders could retract their bid, by doublespending the output used for the bid. This wouldn't be so much a problem if only people bid that actually want to win the auction, but would be a very easy way for the seller to push the price up.
In the end, the auctioneer could just sign the next highest bid then though.
The retracting of bids could be monitored by the auction platform, requiring registration.