Lots of records graded NM- is generally a warning sign for me - I just assume the seller is over-grading and work on the basis that they'll be VG+, especially if they're pre 1970.
I also look for a description of flaws - if it's VG+/VG+ and says something like 'a few hairlines and minor scuffs, name written in biro on rear of sleeve' it indicates the seller has at least looked at the record properly even if it's not been play graded and gives you something to go on.
I contacted a seller once to ask how a VG graded record played. "Like a VG record" came the response
No sale.
I also bear in mind that a lot of sellers aren't professional record dealers, just music fans having a bit of a clear out. So the accuracy of grading will inevitably vary.
As others have said upthread most sellers are happy to resolve any problems (though I've never tried calling one a lying cheating bastard!)