simple answer: NO
no one can dispute a claim - if you go over paypal, they can file a claim only for physical goods, since a website is a digital product, even if the buyer files a claim paypal is not going to rule in his favor. the only case you might lose your money is if you do this on paypal and the buyer pays using his credit card, he might go to his bank and do a chargeback / reversal and you lost your money (and probably your website). if you do this by using escrow, once the money is in your account, nothing can happen to them as the buyer specifically agreed to pay you the $$$ for your website, he had a testing period and accepted the product as it was in the moment of acceptance.
also, from a legal pov, the buyer purchased the website and not the actual rankings or anything like that, so unless you guarantee via a contract of some sort that the ranking won't go down, you don't have anything to worry about.
so in the end, there's just a question of morality but that's your only problem
hope this helps