True, the MARS 'mech it is a later technology than the Coinco,and can probably be set at a higher vending price & it may accept one-dollar coins. bit for simplicity you cant beat the old slug rejector, if the MARS "flight-deck" ( the slug/coin detector) goes, it's a piece or scrap. the Coinco either works or it doesn't.
I just feel that the Coinco is the "correct" style mech for the machine if you're going for originality, other than that, it's six of one & half dozen of the other. ( as to your preference)
So form what I see form my perspective is that you are stuck with the MARS unit unless he's going to pay for the return shipping & send the other one to you for no additional charge, which I don't really seeing him (or her) doing voluntarily,
but if it was an EBay sale, I'd be sure to leave appropriate feed back as to how you feel about the "sale"