Certain years or makers: as with the rifles, the Tulas are popular. Also the earlier the better, with certain makers marks worth more to certain people. Imperials would be the most sought after i think.
Single or double action: I think most singles were converted to double action at some point in time in the refinishing of them. If you find a single I think it would be 1918 or earlier, and that would command a pretty high price tag.
Wood grips or bakelite: I think wood grips were early to mid-war, then bakelite started to pop up. Earlier guns with original wood grips will have a darker colored wood with fine checkering. Refinished guns will have much coarser checkering that is lighter.
Red flags: I would check the function of the gun in general. Make sure the cylinder moves forward to meet the barrel, make sure the hammer/firing pin are complete and not broken, just check the timing in general to make sure it all lines up.

These pistols are built like tanks. Very robust. When you pull back the trigger in double action, it is NOT broken....it really does have that horrendous of a trigger pull. You will give your hands a serious workout if you plan to shoot it in DA very much. SA is easier, but not by a whole lot.

Yeah, the DA is about 30lbs. First time I handled one I thought it was jammed.

I have two, a 1925 CCCP marked one and a 1944 Tula. Of the two the '25 is the better gun. I have a problem with cartridges getting stuck in the '44 and it has timeing issues too (won't shoot DA). My unscientific theory is that the non-war production years were of higher quality.

Well I got to pick between the two they had left, a 1943 Izshevk and a 1940 Tula. I went with the '40 Tula, as the '43 had the rougher wartime finsh. Thw tula had the bakalite grips while the Izshevk had wood; kinda liked the bakelite anyways.

You probably did well with the '40 over the '43. Did you shoot it yet?

Got my 1940 Tula down to the range today for the first time. Very pleasant gun to shoot I must say. Shooting Russian 1977 surplus (thanks AIM), recoil was mild and accuracy was better than I expected with the 72 year old frosted bore ,grooves look like they were lightly sandblasted, the riflings were strong with a good crown. The group in the pic was shot at 10m unrested, single action, from a one knee stance. A little high and left ,but the sight is drifted right so centering it up should put it back on. Case extraction was hit or miss, some fell out on their own, most needed only a slight tap of the rod, while a few I had to pound out with the cleaning rod. The tight ones were never the same chamber so just must be ammo and how things are. Overall I'm very happy and satisfied; I see more ammo in my future. My daughter fell in love with this gun, a ture "surpluser" in the making. Didn't try any double action shots this time. Not sure if anyone already did the translation on the box but it reads, " Revolver Cartridges Caliber 7.62mm" -Thanks

This

The gun version of Red Paper Clip? Hmmm, I see potential here...