If a seller insists on being paid in cash, he's a con artist. Whenever possible, pay by credit card so you can dispute the charges if the goods are misrepresented or never arrive. Or use and escrow agent, who acts as a go-between to receive the merchandise and forward your payment to the seller. Inform the auction site whenever you have a problem. They want to know about sellers who don't deliver or misrepresent their wares.
Services exist that verify and grade products.