I would just pay a different way. Have it shipped first and you have to pay the UPS man with a money order and sign for the shit before he hands it over to you at your house. Or have it shipped to a shipping place that you can pay them when you pick it up so the guy doesnt worry about you getting them free because the place wont give em up til you pay and sign for them and you yourself know right after you hand over money you got your rims right then and there.
Or see if he has an ebay account. Check his feedback. Or tell him to put it on ebay with a buy it now and you win the auction. Ive done that for people who were worried to send me large amounts of cash without some form of security. Saw I had alot of good seller feedback on ebay so id list with buy it now and theyd win it seconds after it was listed.