How do those places that offer to help sell your car work?

[RCCDrew]

So I put my car up for sale on a few sites, just "testing the waters."  I get more calls daily offering to sell my car for a fee. How does that work? Some kind of scam?

[HP2]

You pay them some upfront fee to list on their site, they sell it, take a commission of the sale. Not all are scams, but it all cuts into what you make.