Buy a record cleaner and you will be shocked at what a high % of used vinyl will sound just great.Absolutely required - especially for used vinyl. Some clean them manually - which didn't work for me as I lacked the patience.
When buying used vinyl I do a visual inspection. I personally care less about the jacket than I do the vinyl itself - though I would obviously prefer a record with both in great condition. After cleaning I always put the LP in a new sleeve, and dispose the old (unless it has info printed on it).
I went for about 15 years without playing my personal vinyl collection that I acquired in the '70's - early '80's, and after my kids were raised resurrected my TT and began buying more records. At first I was really disappointed in the sound of my personal collection. I assumed it was all worn out. After buying a VPI 16.5 I learned a good cleaning made them like new. I have bought some fairly gnarly looking $.50 LPs at thrift shops and after cleaning they sound as good as a lot of new vinyl I buy.