I only buy on eBay, but from my experience, the selling price for multiple item auctions is determined by the lowest winning bid. In your example with five items and five bidders, if someone places the highest bid of $5 for only four of the items and someone else bid the starting bid for any number of the items, all five of the items would go for the starting amount. In the end, the highest bidder gets the quantity he wants, and whatever is left over is distributed to the next highest bidder and so on.
If the highest bidder happens to win all of the available items, each of the items would sell at the bidder's max proxy since it is simultaneously both the highest and lowest winning bid. This could be a sore point since a bidder might see that the price has jumped up to whatever he bid, even when other bids are nowhere near that proxy.