Damaged or water wrinkled covers are an indicator the vinyl itself can also be damaged.
First rule of thumb for me when buying used vinyl in a store, is to find a place where there is preferably sunlight, or natural light. I then pullout my LPs of interest in this spot and start one by one critically observing the entire surfaces starting with the outer rim for excessive finger prints and sleeve wear (paper sleeves can really embed paper particles in the grooves here if stored tightly over time with friction applied; sliding a record in an out of a tight storage space). The outer covers will usually exhibit "ring wear" in this case. Then working my way to the dead wax.
I've learned to identify what an "acceptable" scuff looks like (one that makes little to no noise) and what one that will cause distortion looks like (I've scuffed LPs myself at various degrees of intensity to discover the effects it has on play back).
I will not buy an LP that is scratched period. No matter how badly I want it or how long I've waited to find it. This is a tough rule to follow, but one I am firm about after being burned 99% of the time.
I look for luster (the rainbow in the vinyl - natural sunlight helps here). A dull grey LP is less than optimal but if it has no scuffs/scratches (or exceptable scuffs, I'll buy it as most old used vinyl falls in this category.) Even dull grey vinyl can get it's groove (luster) back after a good washing with a good RCM.
I typically wont buy a pre-cleaned LP (these will show luster most times) because I'm particular about the fluids I use and can not vouch for the previous owners choice (did they use an alcohol base clearer (don't like them) and/or methods. Also, knowing the effort to properly clean 1 record, I find it impossible to believe someone selling pre-washed LPs takes the time and effort to properly clean them with good fluids. Look for fluids in the grooves (looks milky white) quick hand washed records are easily identifiable because they exhibit a noticeable "whipping" surface film on the vinyl.
Good luck! Buying and finding used vinyl treasures is a favorite activity of mine in this hobby.