When asking "what payment methods you can use on flippa" I assume you want to know what payment methods you can use when PAYING flippa (i.e. their service fees)? In this case, the answer is paypal and credit card.
If you were talking about receiving money from the buyer of your site, though, then it's completely up to you. Flippa do NOT mediate any financial transactions so it's 100% up to you and the buyer to figure out how to transfer the funds. While PayPal and Escrow are most popular options, many mid- to high-end sites are paid by wire and some people even use Western Union.