I have been collecting vinyl for several years now, and Discogs has been my go-to for finding less-common to rare vinyl.
Firstly, their database is very expansive, so it is usually quite easy to find a specific pressing of a given album, and I love their setup to build a "my collection" list of my personal collection, as well as a "my wantlist" that includes the option of having them email you when items I'm looking for show up online...It's not perfect, as you can't set a price limit so that you're only notified if an item shows up for sale within a given price range, and it isn't great if you include readily available items in your wishlist, but for items that only show up occasionally it's an absolute blessing!
I don't know what it's like to be a seller on discogs, but as a buyer I've been very happy. I have personally purchased several dozen different records on discogs, and have only ever once received an item that I was less-than-satisfied with (it was a low-value item for a gift, was listed as VG+ and was more like G+). All my other interactions have been great! Note that most sellers grade their vinyl visually, so if you are incredibly stringent on what you expect for NM/VG+ then contact the seller to ask questions first, although many sellers actually grade their vinyl low to prevent complaints from the pickiest of collectors - most of the VG+ I've purchased has been closer to NM.
Obviously this is a marketplace, so you need to use some common sense when purchasing, but sellers need to have their account linked to a confirmed PayPal account so it's way safer than, say, craigslist.
I would highly recommend purchasing from here, or at least using their database to generate a "my collection".