I've taken a look at Penny Auction sites, and from what I've read, using them is almost like gambling. How they work is when you bid on an item, that bid costs a fixed amount of money, whether you bit $0.01 or $50.00. Also, the bid seems to go on for quite a long time, as even if the clock has less than 10 seconds left on it, when somebody else bids the clock goes back up. Generally, this can go on for a while.
For example, if you see a 600D selling for $30, there will probably be quite a few people interested, let's say about 30 people. The 600D is listed on a site where one bid costs $1. If there's 10 seconds left on the bidding clock and you bid another cent, putting the total bidding up to $30.01, you've essentially already spend a dollar just for the right to bid, whether you will win the auction or not. The bidding continues on for another 30 minutes, and now it's up to $100. A 600D for $100 is still a good deal, but you've bid $0.01 80 times now throughout the entire auction. You've now essentially spent $80! In the end, the bidding goes up to $150, and you bid another 70 times. However, in the end somebody else outbid you and ended up with that 600D, selling for $170. Even though you didn't win, you've still spent $150 on bidding alone, even if each one of your bids was for only $0.01. And, because there were 30 people that were bidding, if each one bid 150 times like you, the bidding site made a total of $4500 off an item worth $700. Subtract that $700 from the $4500, and they've essentially made $3800 off of the bidders.
The only winners here are the bid site, as they've made an enormous profit in only 30 minutes, and the winner of the item who effectively paid his/her $150 for the bids and $170 for the item (a total of $320). But, are you really willing to risk $150 for a 1/30 chance of winning a $700 item for some half the price? That's why it's a gamble, you're very unlikely to win, and the one almost certain to win is the bidding site. The odd time you may get a good deal, but it's rare.
Gear: 7 Nikon Nikkor AI-S and AF-S lenses, SB-700 flash, Nikon D7000, Nikon FM, variety of accessories
"There are no rules for good photographs, there are only good photographs."
- Ansel Adams