scam penny auction?

How do you tell if a penny auction site is a scam or not?

First off, what constitutes a scam? A scam would be if nobody every wins an auction, or items won are never shipped.

I found this penny auction watch site a good source of information:

http://pennyauctionwatch.com

Here are some sites that look legit to me and that seem not to use any shady practices:

http://swoopo.com

http://bidrodeo.com

Are there others? Please let me know in the comments section.

thanks!