I've been using Discogs almost exclusively for used vinyl for the last several years. Maybe I've been lucky, but I can't remember the last time I got a incorrectly grading LP using Discogs. I think professional sellers are getting much better at grading. I check feedback carefully looking for comments indicating accurate grading. I also prefer sellers who give more detail about condition. If an album is rated VG+ and they describe the level of noise, for example "light crackle during quieter parts", you have a good idea of what you are getting. I try to avoid sellers with very few ratings unless the prices are very low.