Unless you are talking a about price
that is low enough that you could lose it all and only be slightly inconvenienced, then you definitely want a formal, written contract. For me, when I bought a small sailing dinghy
for $800, it was just a handshake and hand over the cash. When I spent $2,500 on a runabout I wrote up a formal bill of sale
and insisted on getting the sellers signature on it. Much more than that and I would want a full-on contract, with inspections, a sea-trial, and options for backing out.
And how much you trust the seller doesn't have anything to do with it. I trust my wife, my mother, and my father more than anyone else in the world, but when we play pinochle I still cut the cards!