I'll go out on a limb a bit. The boxes are rough, the ammo is gonna be worth about as much to you as it's replacement in retail. By the time you find a guy that wants them, and , if he's a dealer, you have to account for that he has to make something on the deal... Probably as well to just use them and see if anyone wants the boxes.
That said, I'm not an ammo collector and I don't stay caught up on the intricacies of which variation or print run of boxes is uber rare, or anything like that, so really, the best thing you can do is to trundle off to the Cartridge Collectors Show and talk to the folks there, or talk to the Cartridge guys at a Gun Show.
So, sort of a non-answer, but that's what I usually do if I end up with some old ammo in decent shape, esp. if I have not got a gun for it.
Cheers
Trev