If it were a JD unit it would probably have a tag on the front like the 80,50 do. I dont think its a JD but could be wrong. Research / Google JD carts and see what you come up with . I have to agree with Don right now though.
I'm saying it's not a John Deere cart. It resembles a model 10 cart, however the bottom of the bed sides don't appear to be rolled under like a model 10 would be, and I don't think the top front corners of the bed rails are shaped like that. If the seller is so sure of it being a John Deere cart, then tell him to prove it by showing you the serial number tag. The only other way of proving it is a Deere cart, is to remove one of the tires to see if there is a John Deere part number stamped on the inside of the rim.