Upload & Sell: Off
I'd sell it and give the money to a charity or find a guy who got ripped off here on the boards and pay it forward.
Your obligation to the seller ended when he tried to defraud you and ran off without contact. Obviously he feels guilty enough that he is laying as low as possible right now. Still, it is mighty decent of you to offer the return, but it should be at his expense and you should give him 90 days maximum. If you don't hear from him after 100 days you should assume his interest in the lens is no longer valid.
If you want to do this on the 'up and up', simply charge him storage fees, say $10 a day. If he doesn't claim it by the time the fees add up to the value of the lens then you can attach the fees to the value of the item and liquidate it. If he wants it back, charge the remainder of accumulated fees and shipping to have it released to him.