What do I do about this dodgy Ebay seller?

are you serious? nobody will know what one is, unless they are into bikes. his listing is accurate, technically, but it does not explain that this is therefore dangerous to ride!
No one reading the auction will think that someone would have the gall to sell something they knew to be dangerous, and will assume the 'lockring' isn't a big deal, as he said.
Buyer beware my fucking arse.

Yes I am quite serious.

"Car for sale, no distributor cap"

If you were interested in buying a car and saw this, would you not question SOMEONE about whether or not the distributor cap was necessary for safe or effective operation? If not, you are a fool.

Listing is accurate. Buyer beware.