As Sabre mentioned, there are always risk when you bought a developed site. Because services like Flippa are not acting like a middle-man. There are so many things that you should get from the owner, like domain transfer, files, databases, e-mails, social media accounts and none of them are owned by the services, they're still belong to owner. So there is a huge risk on that.
Btw, there are services like tweaky.com that helps to transfer website files, but they're so primitive for now.
I think that the middle-man-services like Flippa should take all the risks for buyers. After the agreement process, seller should transfer files to the service and buyer should transfer money to the service. After the website transferring process done, service should send money to the sellers. This is the solution.