Yes. All real-world business transactions involve formal contracts, and Flippa auctions and sales are no different.
For your safety, we recommend that you use a contract of sale.
We understand that the cost of getting a lawyer to draw up a contract can be prohibitive, so we've created a sample contract that may help you in your Flippa transactions.
A written contract of sale helps both buyer and seller know what they're getting—and what their obligations are. It can provide peace of mind, especially in cases where the parties to the transaction live on the opposite sides of the world, and may never meet in person.
Obviously, this sample contract comes with no guarantees. Download it, have a read, and see if you think it will work for you.