Well if he sells you an Antminer S9 and it breaks, you can get a full refund in 6 months if you pay by Paypal. He will even have to pay shipping both ways. Even if the doesn't break, difficulty can quadruple in 6 months and you can get a full refund.
If you pay cash, bank deposit, btc, then they are safe against refunds or exchanges which the S9 has a huge probability of failure.
Alot of auctions on eBay are like this, they were probably burnt before and don't want to take that risk again. So they only accept methods that are non-refundable, but I am sure they will ship you the miner if you paid.
There's guys that even mine miners to death and return them or return bricks to the seller, and in most cases the seller can't do anything about it. It makes sense for these people to price things higher as they're going through such risks constantly. Of course like you just said there's also guys that scam, but going with a trusted buyer AND USING ESCROW can prevent such risks on your side if you choose to buy with bitcoin.