To avoid scammers altogether, put your rig with a reputable consignment dealer. We've just done that. The dealer charges a flat fee of $595 regardless of the price it sells for and will do necessary minor repairs while on their lot, with quotes ahead of time for approval. He waits until it sells, then deducts the overall cost of repairs from what he sold it for. I have approval power over any offer.
We anticipated getting maybe $20,000 for ours in a private sale. The dealer thinks he can get $6000 more.
We had a scammer show up at the house asking if we were considering selling. I said yes, and he inspected the rig carefully. He then offered $12,500! Not even knowing what the consignment guy thought about it, I told the scammer to get stuffed.