I buy alot of used lps off ebay (Amazon is another good source). IMO, the safest is to buy from a large store that grades using goldmine standards. Next, buy only M or NM graded lps. If its not M or NM I ask for return or refund or partial refund. It is nearly always honored. When an lp is graded VG or EX or great shape, it leaves too much room for interpretation. But M or NM does not leave any such room. Most vendors on Ebay do not want to risk a negative feedback over $10 or $15. So th safest is to buy that way. But I have found some great deals on lps from individuals who are unloading their collections. You have to take a chance on the quality of th grading, but snagging a few great titles for a few bucks is strong incentive to ignor the occational bad buy. Cheers and good luck.
