A Reserve Auction is one in which the seller sets a minimum price at which he/she is obligated to sell his/her product or service. Typically, these auctions start at a very low bidding price. The Reserve price is always hidden.
How The Reserve Auction Scam works:
Let's call the eBay scammer Ed.
Ed lists an auction for a plasma TV with a Reserve Price of $6000 (yes, I know that's ridiculously high, but that's the whole point of the scam. Bear with me.). He starts the bidding at .99 cents. The auction is a 3 day auction. There are 6 bidders and Sue bids the highest at $850. The next highest bidder is Jack at $825. The third highest bidder is Mark at $800. Sue did not officially win the auction because the Reserve price was not met. Ed emails Sue and asks her if she wants the item anyway. Sue is delighted. Ed tells her to send a payment of $850 to PO BOX 123, Any City, US 12345 in the form of a money order. Ed then emails Jack and tells Jack that Sue decided against the item. Jack is delighted and sends Ed a money order in the amount of $825. Ed then emails Mark and informs Mark that neither Sue nor Jack wanted the item. Mark is, of course, elated!
All three buyers sit and wait for weeks for their plasma TVs. They contact the seller and receive no response. They contact eBay and eBay informs them that their purchases are not covered because the transaction was never completed via eBay. Even if the transaction was completed via eBay, all three buyers used money orders to pay for their purchase. eBay's Buyer Protection does not cover money order purchases.
Now, Reserve Auctions aren't always used to scam buyers, but I'd certainly be leery of any email communication from a seller who suddenly wants to sell you an item he/she originally didn't want to part with unless he got a certain price for it. Contact others who bid on the item and ask them if they've had contact with the seller also. As long as the seller didn't make the auction private, you should be able to view the list of bidders easily.