If you look at the tire pic, it looks like a fresh application of undercoat in the fender well.
I noticed this as well. This is always a HUGE red flag for me for any northern truck.
Honestly, my guess is either an unreported accident repair, or a rollback, and/or a more serious rust issue that's been masked with a quick and cheap coat of rubberized undercoating. Worth a look if it's not a long drive, but I wouldn't hold out much hope and would be looking this over with a fine toothed comb.