LMK if you need an escrow. I'll do this one for free.
Will do.
If he's a scammer, he'll never take me up on the offer.
If he's legit, he's going to have a hard time finding a better offer than mine.
Has it occurred to you that these were coins scammed from other people? They're now being laundered, so of course he wants the deal to go through. You're not getting scammed directly, but you are a part of his scheme to get clean money.
Whether or not you care about the BTC originating from illicit activity is another matter.
Bitcoin is fungible so regardless of where it originated is not reliviant, although buying from that specific address could, in theory attract attention from law enforcement and result in the loss of privacy if his identity is associated with however he sells the bitcoin when he goes on to sell/spend it.
As mentioned above marc might have well laundered the bitcoin via bitmixer or one of many casinos that can/will effectively launder funds for you.