What do I look for in used records?

I like the local used record stores here around Lancaster, Pa., and the Pa.Record Expo that meets here monthly. I also like the guys that grade the records and I go for near mint, mint or VG+. Having said that, I look for a record that has no visible scratches. Dirt is ok, as I can clean it off. I went to the local Goodwill, and got some as a result of reading posts as well. Got a record that looked as though it met my criteria, cleaned it and lo and behold it sounded terrible. Pitched it. Also, if the person who owned the record previously put in a new sleeve, I tend to feel real good about it. I recently bought about 7 albums for a quarter a piece, with the covers pretty beat up. But the records were in new sleeves or at least looked good, and they sounded great(Record Expo). So, I haven't answered your question specifically, but I don't know the answer to your question specifically. Find some places you trust and enjoy yourself. Check other posts for cleaning procedures.

Tough question to answer.
Buy what you want to try! Most of the LPs in the thrift shops are not going to be "valuable" except to the buyer.
I have found stuff like STEREO shaded dogs (mono are nice but cheap resells)(the dead wax codes start with 1s, so a 79s is pretty much a later issue) and Merc 90000 series. six eye Columbia or later two eye Columbia (Columbia dead wax stampers start 1A then progress throught the alpha to 1aj etc then 2a... so an early pressing is more desirable, That means even if you have a title, look at more to find an earlier pressing... a GREAT way to spend/waste time)
The other labels matrix numbers are a secret art know only by a small cabal of LP mavens... not me sadly.
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Condition is everything: a scratch makes a LP WORTHLESS. (unless it is a rare and valuable one say a 'Blue Note jazz LP') Dirt is not a problem, except it can hid scratches. A DULL surface usually indicates the LP is very worn, especially if dull more towards the center. (but some labels are pressed with an even but dull finish.)
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A badly worn jacket should NOT be passed up automatically... it can and often does hid a very good condition LP. and conversly, a great jacket may contain a dreadful ruined LP.
Stereo is rare before the mid-60's and any LP from before then will certainly be MONO. Avoid 'electronically rechanneled for stereo" they suck... get the original mono instead.
Never buy any without checking that the label and cover match. And folks DO buy Lps just to get a better cover.
LPs without the barcode on the back are earlier than ones with a barcode... and more collectable.
Remember that scuzzy LP may be the only copy you will ever see, or it may be super common... experience is the teacher.
And pass on those Mitch Miller singfests, so other seachers have something to slog through too!

To add a little bit more to the above posts, the first mass produced stereo recordings were available in the late 1950's. I believe they were predominately classical releases (someone correct me if I'm wrong). The collectors go primarily for the RCA, Mercury, and London records (from this era). As for identifying the stereo versions the RCA's are of the LSC-xxxx series (Living Stereo) and the Mercury's are of the SR90xxx series (Living Presence). I'm not so sure about the London's, I know the early issues are identified by their light blue back (of the jacket), but I don't know if they are all stereo. As for the mono releases, the RCA are identified by LM xxxx and the Mercury are identified by MG09xxx. With respect to the album Wdi found, it is a mono and probably predates the first stereo releases (1957?).