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.

"PAYMENT IS TO BE  BITCOIN/BTC , CASH  IN PERSON, OR BANK DEPO (NO CHECKS). NOTE WE WILL REFUND ANY PAYPAL PAYMENT OR CC PAYMENT ATTEMPTED  FOR THIS AUCTION."

If you pay cash, bank deposit, btc, then they are safe against refunds or exchanges which the S9 has a huge probability of failure.

None of these forms of payment are allowed on Ebay. You MUST offer at least one of the electronic forms of payment such as PayPal or CC

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.