First thing I would look for is matching numbers on the barrel and bolt. Matching magazine floorplates aren't that important for a shooter. If you find one where the numbers match (not forced match), meaning the numbers haven't been lined-out or ground off (if they have been, it's a "forced match"). Generally, all-matching rifles are smoother in action than those force-matched.
Next thing is inspect the bores. If you don't have a bore light, the gun shop usually will, failing that, just remove the bolt (open the bolt, hold back the trigger, and it will slide right out) aim the barrel at a light source and you'll be able to see it well.
As noted above, the grooves will often be dark and rough. Look at the lands, they should be shiny and not pitted. Then examine the muzzle. As noted, look to see if the crown is gouged and/or the rifling is heavily worn at the end of the barrel. Counterbored barrels generally will shoot better than non-counterbored barrels, though aren't as "collectable" if that matters to you.
Other than that, just try to see fi the stock has any major splits or such that would require repair.
Mosins, like most other Soviet weapons are extremely tough and reliable. A 91/30 with a good bore will shoot pretty darn well. Mopst horror stories come from people who bought rifles with shot-out barrels, and pitted and corroded chambers. One reason I personally prefer to buy guns from a gun shop rather than ordering it. I get to inspect and handpick what I want. Plus, as noted, after you factor in the shipping and transfer fees, plus the hassel of maintaining a C&R bound book, you really don't pay much more for the gun shop model, and you save a lot of hassle. BTW $120 for a very good condition Mosin is right on from a gun shop.
Personal impressions of a Mosin 91/30:
They're sleek, balance well, shoot well, and are mean as hell looking (it IS after all, a five and a half foot long rifle with the bayonet attached). Oh yes, shoot it with the bayonet attached, that's how they were sighted in.
I prefer Mosins to Mausers.