+1 @mijostyn For experimentation I took a few of my older audiophile records to my dealer who recommended a record cleaning machine and we tested them on a system about at the level of mine to see if cleaning actually made them sound better. I handle records very carefully, keep them in rice paper sleeves and the jackets in polyvinyl sleeves, keep them vertical, etc. So they are clean. I use a micro fiber anti static brush - Audioquest just improved theirs a bit - its $30 and I swipe before each playing.
I could not hear any difference between the pre and post cleaned records, nor could the dealer. That is what is important after all. The sales pitch stopped. I rarely buy used records - only when there is something I really want that is not available. If you are buying used records and the place doesn't clean them on a good machine, I guess you should get one. Most really good records become available at some point, you just have to be on the look out since they can sell out pretty quickly. Rumors just became available on a 45 and it sounds great. Blew away my Nautilius Recording of it. With use record prices rising so quickly, many times new ones are well worth it. Especially Blue Notes for $25.
Acoustic Sounds, Music Direct and Elusive disc are all very good sellers (AS & MD are also manufacturers), although AS is selling so much it takes them up to a week (or more if a blockbuster just came out) to ship stuff out these days. Amazon recently improved their packaging, just be careful that you are getting the pressing you want.