All these sites have a purpose. I used AG , USAM, and e-bay. For the most part they all have saved me well.
The only bad experience was that the buyer received the product and told me that it wasn't working. After trouble shooting several times over the phone it turned out that he just didn't like the unit. I received the unit and bought it to the manufacturer .
As suspected they gave it a clean bill of health. However he lied to PayPal and I was canceled . PayPal not only paid him direct but held on to all my money in the account for 6 months.
With all these sites the more communication between the buyers and sellers you have a better chance of a mutual outcome. I have become Web friends with buyers and sellers.