Look for these to be shut down soon. I looked into this before and have a domain giftcardbidding.com but after researching it I find its more illegal than grey. The software provoder for most penny auctions (the real name of these) also sells a bot to bid against real people. These are 99% scams, plain and simple.
When you read the terms of these sites you see that A, most are not in the US, and b, the money that is to be paid for the item, The bidded amount is dontated to charity.. When we give to charity its a sure sign they are avoiding somthing.
You pay for bids, so a person can bid for an item and not have anything in return, hence gambling to be the last bidder. Its a giant money making idea, but it will soon be illegal to run if its not already.