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.