I've been using Discogs to buy and sell for 7 years with very very few issues. I'm careful to check sellers have a good feedback ratio, an established sales history, and always use Paypal so I'm covered by their buyer guarantee if not happy with the grading and the seller doesn't co-operate. Out of 150 purchases I've been unhappy with 2, and returned only 1 as the other was borderline and no big deal. There are bad sellers out there, on any site, but it does surprise me when as a seller I have 1700+ positive feedback, 3 negatives from people who had a problem with the postal service not the record, and were provided with a full refund at my expense (as directed by Paypal seller agreement it is my responsibility to make sure the record is delivered, or refund) just how many people are super suspicious when buying from me purely because I sell on Discogs purely because they have had bad experiences with others. How on earth would/could I have a 99.8% positive over 7 years if I was a rip off merchant? Buyers need to remember Discogs is an open market for 3rd party sellers so if you use your due diligence (read: you NEED to) you should have few problems.
Aside from the buying and selling aspect their database is really annoying! Open for anyone to come and sabotage a release, so you list a copy of a certain record for sale, or purchase a copy and then find some fool had gone and changed the entry from CD to LP, or original to reissue, nobody spotted the edit was b/s and you now sold/bought something you didn't want. That really annoys the cr*p out of me hence the 3 star rating.