I think if we get another two great seasons out of Auba then that's two more than we've ever gotten from any striker since Henry left us. Resale value isn't everything, and I feel it shouldn't matter that much to the fans - ideally you wouldn't want your best players to be sold.
I would have reluctantly let him go earlier this summer if it meant using the money to sign a straight replacement with more years left in the tank, like Jovic or something, but I'd keep him regardless of the trade now. Also, giving United another golden boot winner is just not on. I'd sooner see us relegated than deal with that virus of a club.