Most eBay sellers have PayPal only as the default provided to them by eBay when they list the item. It's just too much trouble to go to the bother of adding other payment methods - I don't, for example, but if someone offered cash I'd be perfectly happy to accept it.
If you refuse to pay for an item you've won on eBay you are legally in breach of contract. For a few quid it's not worth worrying about. As a seller you simply report the buyer as a bad payer and get a full listing refund so you have no costs to bear. Pretty simple to me.