I just bought a used Squier MB5 from Craigslist a couple of days ago- body in good shape, but the neck was bowed like crazy! Strings were about 1/2" inch high at the middle of the bass- made it extremely tough to try out. I looked at the truss rod cover screw and saw that it had never been touched. Good sign that the truss rod had not been touched, and overadjusted by the owner- (he actually did not know how to fix the neck, so did not touch it and decided to sell it) . I thought I'd take the risk (for $100)- I was feeling lucky. After some adjustment, and noticing that there didn't seem to be any tension on the truss rod at all (did the factory forget something?) tightened it and I got it to a much more realistic (actually normal ) string height. I did some fine adjusting earlier today and got it very low which is the way I like to play it. Seems to hold and play fine.
I would probably point that neck out to the pawn shop and that you're taking a chance on perhaps buying a bass with a broken truss rod, offer to pay $40.
You can then try to adjust it, if the truss rod is indeed broken, you can always replace the neck and maybe even a fretless one at that.