Nothing is really iron-clad, but a simple statement saying that you are selling the equipment "as-is," and that the buyer is responsible for moving, disassembling, reassembling, and using the structure to/at another location and agrees to be assuming all possible risks to health associated with this moving, disassembling, reassembling, and using. Have them sign and date -- better to have it notarized, but probably not feasible unless, say, a neighbor is a notary. Include an acknowledgment of receipt of the cash/check/etc. and treat and give it like a signed receipt.
Good luck!
