There is a wealth of great info here. One thing I am a little surprised no one has mentioned is the fingernail test for scratches on used LP's. Gently run your fingernail across the scratch - if you can feel it, you will certainly hear it, and do not purchase the LP. If you can't feel it, this is not a guarantee that you won't hear it, but there is a good chance that you won't.
I have found this fingernail test to be much more reliable than visual inspection. Many times I have cleaned an LP that looked fine visually and then turned out to have groove damage that was not visible, even looking at it after I knew the location of the damage.
Visual inspection will also almost never reveal whether or not an LP has a great deal of surface noise from the pressing. I can't tell you how many times I have been disappointed by the RCA "shaded dogs" in this particular respect. Sometimes an LP was just pressed badly, and no amount of cleaning will fix it.
Speaking of cleaning, I agree that a RCM is a must. I also highly recommend the AIVS fluids - I have had some very good results using their three-step process.
I have found this fingernail test to be much more reliable than visual inspection. Many times I have cleaned an LP that looked fine visually and then turned out to have groove damage that was not visible, even looking at it after I knew the location of the damage.
Visual inspection will also almost never reveal whether or not an LP has a great deal of surface noise from the pressing. I can't tell you how many times I have been disappointed by the RCA "shaded dogs" in this particular respect. Sometimes an LP was just pressed badly, and no amount of cleaning will fix it.
Speaking of cleaning, I agree that a RCM is a must. I also highly recommend the AIVS fluids - I have had some very good results using their three-step process.