Most dealers at gun shows don't bother putting any grading on the guns they have for sale. You like the gun and the price, you buy it.
NRA is the standard, but it take a lot of experience to really get any sort of reliable grading that is more than just subjective.
For a shooter, function and bore is more important the finish. For a collector, the bore may be a little less important than external finish.
Mosin-- Watch out for counterbored barrels. They may shoot ok, but something to be aware of if that would bother you. Finnish Mosins are nicer than Russian, but will cost more. Hex receivers are more prized than round, and Tula is more prized than Ishevsk. Most common models are M91/30 (long rifle with socket bayonet) and M44 (carbine with attached bayonet).
k98-- Unless you want a serious collector, a Russian capture is the way to go. Try to get one with Waffenamt stamps still intact. They will not have matching parts. Some of them will have force-matching with electropencil.
Swede Mausers are among the best shooters, will will cost a lot more than your average Russian capture. A Vet Bring-back with all-matching parts can go for $800+, so you may not want to do that until you have a better idea of what to look for.