My father and my uncle used to DJ, so I always had CD players and 1200's in my room growing up. They had a hefty collection of mainstream music on CD and vinyl so I got into the formats early on. I didn't really start collecting on my own until early 2004, and from there's it's been full on hoarding without any pauses.
I often buy collections in bulk from sellers (mostly) in Stockholm. It's a great way to get to know new and old music, and I usually keep what I want and sell the rest. My main focus is collecting old (mostly 90s) techno, electro, house, idm, jungle, acid, ambient and similar styles.
On Discogs and other sources, I usually buy mid-90s compilations and cd singles to get full versions of specific tracks. Even if I have the complete vinyl version, I prefer having the full length version in .wav format ripped from CD for my own library.
When I purchase files online, nowadays, it's only .wav. If I'm going to pay for a digital file, I want the full thing, not a lossy mp3. I'm no audiophile, and I probably won't hear any difference, but I want to build a complete .wav collection of every track I like. It will never be complete, and that's a challenge I really enjoy. The library is linked to my CDJs so I can play the tracks without digging out the CD from the shelves.
I'm trying to buy less physical and more digital at the moment, because space is limited. Not everything's listed on Discogs yet, but I would estimate the collection to north of 8000 physical units.