Upload & Sell: Off
That's an awfully broad brush you're painting with. If your experience was representative of reality- and thankfully it isn't- the majority of sellers and buyers on this site, including me, would be scammers.
In my case, with CL, it has been the case that those who mention PayPal are scammers. I, therefore, ask for a certified check if they are not face to face. Then, I take the check to the local affiliated bank and I ask them if it is a legal check from their bank. If they say yes and cash it for me, and I have all the particulars from the customer, and I have spoken on the phone to them, then I send the lens or whatever.