Upload & Sell: Off
I would say you have fulfilled your end of the bargain. You don't have to wait forever in case he finally decides he wants to talk to you again.
I'd give it a month and a few more emails. Give him a clear deadline: "If you do not respond by June 1st, I will assume you no longer want the lens." Make sure you've documented what you've sent him, and then do with the lens what you want.
Use it, donate it, whatever. I guess I personally would feel a little sketchy about selling it for a profit but even that: you have held up your end of the bargain, you do not have some moral responsibility to harass him to ensure he gets the lens back if he's not going to respond to you.