Ex-snipers are Russian PU, PE or PEM rifles that had the scopes and bent bolts taken off to put on other guns. Sometimes, they have better triggers, however the Soviets are rumored to also have taken the good triggers off of the ex snipers and put them on new ones.
USUALLY, ex-snipers are more accurate. They usually have a stock repair on the left side of the stock where the scope mount was. These guns were restored to regular 91/30 specs but again were still hand selected for their accuracy.
If you want a regular M-91/30, look for:
1. A hex recievered rifle.
2. A "K in a circle" stamped on the barrel. This mark is believed to be an accuracy mark put on rifles that achieved higher than standard accuracy.
3. Non war dates usually have better finish.
4. TRY to find one with a stock numbered to the rifle, and with the mag floorplate not lined out and renumbered. This would suggest a non-rearsenaled gun which is worth more than a standard refurbished gun.
5. If you can find a M-91, get one. These look like the standard M-91/30, but have an unprotected front sight (no globe around it) and some are Dragoon style, which means their rear sights look like a zig-zag and are usually marked in Arshnis, an old Soviet measurement based on the human pace). Many M-91 rifles were brought to M-91/30 specs by changing the front sight and re-marking the rear sight in Meters if it was a Dragoon... you can tell this was done by the date on the barrel. However, if it is "SA" stamped, it may very well have been a barrel change done by the Finns.
Other desireable markings:
1. "MO" stamped on the barrel. This is the Soviet ministry of defense mark.
There are many other markings... thousands, litterally. I can only go over some of the highlighted ones, like SA or MO or K in a circle. Also, another semi-unsual one is the Y in a circle. This is not a known mark, most experts think it is a black-powder proof, but it shows up on rifles as late as 1947 which makes me very leery of that theory.
ETA: here is a more complete overview of the markings
http://www.russian-mosin-nagant.com/arsenalmarks.htm
ETA2: the sticky bolt issue can be fixed by using a .410 phosphorous-bronze brush in a drill with a patch of Flitz or M.A.A.S. polish. Use the .410 brush with the polish as a chamber brush, running the drill both directions on the chamber for about 30 seconds. Take off the dirty patch and add a clean one, and keep going untill the patches come out relatively clean. This usually fixes the sticky bolt issue.