To infer that all used deals are suspect and disaster ridden is silly,and not rooted in reality.The apparent attitude of the thread starter is unusual to say the least.To assume that only new purchases are safe is false security too,witness the current problems at Brison.
There are many things you can do to ensure a fair exchange starting with dealing only with people you know,or dealing with high input count members of R/C sites like this one.,or those on good guy lists.The trend to photos helps too.....
I have bought and sold many glow and gas engines on the net,and have no complaints on any I have bought and no returns on any I sold.I describe the engine Im selling as acurately as I can,ask a fair price,and after the price is agreed to ,live up to my part of the bargain to the letter.I always throw in a little extra too like a prop after the fact,and follow up the sale with a howgozit note a few weeks later.
Incidentally,every G62 I ever saw,including mine had those scrape marks.......what do you expect? Theres about 10 thou clearance between the magnets and coil and dirt and dust is stirred up,and the magnets will pick up any ferrous material, every time you run the engine on the ground,so of course it gets marked up......and it means nothing and has no affect on the performance of the engine......