You will need a bill of sale, a contract reflecting the warranties (if any), a permit transfer form (if applicable), and an assignment of ground lease and ad leases (and an estopple certificate is a good idea, too). I’m no lawyer, and I recommend that you definitely get good legal counsel to guide you through whatever you need for the state that you are in.
The most overlooked item, with disastrous effect, is the permit transfer form. In some states, like Texas, if you buy a sign and do not get the approved permit transfer form filled out and signed and sent in, the permit effectively dies and you have to take the sign down. You also have to be a licensed sign operator in some states in order to take assignment of a permit. You definitely do not want to make that mistake. Call your state’s department of transportation to start the fact finding process. Check both state and local regulations.