I buy and sell on discogs.
Buying: I have a wishlist of things I want. A mixture of things I used to have on CD and would love on vinyl, and things I have never had but would like to have a physical copy of. I don’t live anywhere near any real record shops, so discogs is realistically the only place I’m going to get this stuff. Because I’m a terrible proscrastinator I regularly check the marketplace for newly listed wishlist items. Because postage can be expensive, if I see something crop up, I always check a seller’s other inventory to see if I can combine shipping for cheaper per-item postage. This often leads to interesting new discoveries of things I never knew I wanted / never knew existed.
Selling: I sell for two reasons.
a) Clearing out stuff from my personal collection that I no longer listen to, and selling it at the lower end of the going rate (so that it’s priced to sell, but still worth the effort of listing / packing / going to the post office)
b) Seeing things cheap - charity shops, car boot sales etc - and selling them on for a mark-up, but still to be at the lower end of the going rate. I see this as providing a redistribution service. I’m putting the effort into crate digging (which I admittedly love doing), and finding something that someone else might want, and then charging them a fair price for it.
In both cases, I always aim to have the cheapest available copy for the item’s condition.
What I have sometimes done: seen something cheap on Amazon (because often Amazon does odd things with their automated pricing), bought two copies of it, one to keep, and the other to sell on discogs for enough to cover the cost of both of them. So I’ve effectively got something for free.
What I’ve never done, and don’t think I would ever do, because it leaves an unpleasant taste: buy multiple copies of a brand new limited release in order to sell at vastly above the initial sale price.