I've had that problem a lot when I go to look at classic cars mostly...the seller always indicates "surface rust", which turns out to be "holes so large you can fit your head through them" (mostly New Orleans, even worse post Katrina). So I do have a rule to never look at a car after dark, for obvious reasons...even though I've gotten pretty good at playing that game.
But this, being a tool box, is a little different story. Did you just hand the guy cash and throw them in the truck? Seems like a two minute inspection with a flashlight or the headlights of your new truck would have revealed these problems.
I did on the snap-on box but the slides didn't seem that bad and the body was in great shape bedsides a few dings. If it doesn't rain today i will take it out and see how bad it really is and how much work it needs. Either way for $120 it was a great price! I just didn't want another project at this time