I think, for as much as I love VGM, I'd have trouble coming up with another 500 albums I would want to own. 100? Yes, definitely. I need to fill some holes in. 200? Okay, throw a ton of rarities/impulse buys/curiosities in there and I'll probably be there in 3-4 years. 300? Really stretching it. Maybe if we start counting up all the individual Game Sound Museum singles (ha!) and throw in all the cheapo Falcom/Twinbee Paradise/Tokimeki drama CD's that can be had for a song (double ha!), I can really bloat my count up. 400? 500? No way. Even with an unlimited amount of disposable income, including digital albums I buy and counting anything that gets sold between now and then, I'd have trouble with that.
I guess that's the balance of being both a collector and a fan of the music.
I find it both easier and less stressful to just buy new stuff as it comes out. Unless I'm really bored or really interested in something I don't even bother checking secondary marketplaces, it just takes too much dedication and time for me to bother.
I average about an order a month from CDJapan and I went through and found 13 orders I made which consisted of 38 total albums in 2015. I buy mostly based on artist. Everything from Hisaishi. Almost everything from SQEX, Sakimoto, Sugiyama, Kanno, Hamauzu, etc. I'm very keen on box sets just because of how convenient they are in amassing a great quantity of source material in one purchase instead of having to hunt down copies of everything (again, time, dedication). I bought the Gust box set just because it was easy despite never hearing much of the music from any of them (except the one Kikuta had a part in).
In real slow months or quarters where there just isn't anything interesting coming out, I'll randomly purchase things based on popularity, recommendation, exploration or just complete random. I bought the Gargantia on the Verdurous Planet soundtrack because it was a slow period and I just liked the cover and it's been one of my favorite albums of the last 5 years. I bought the Taira no Kiyomori "box" (5cds) because I saw someone recommend it and listened to some samples on YouTube and I now *LOVE* it. I bought the Attack on Titan movie score just because that series seemed to be popular at the time and I'm not really a big fan of it at all. I bought the Death Note movie OST around the same time because I have a soft spot for Takayuki Hattori... the list goes on.
I prefer it this way because I'm less likely to have to go back in 5, 10 years and be like "dang, really missed an opportunity to get that when it was current so now i have to look on secondary markets". Seems like a pretty good methodology if you aren't too worried about fiscal responsibility and generally know what/who you like.
A vast majority of items in my collection are in the orchestrated or arranged style, or just classic nostalgia SNES RPG era stuff. Mostly the "cinematic" type stuff. I don't really like much of the rock, fusion arrange stuff and I pretty much detest some of the worldly instrumentation styles like Celtic or some of the shit on the SQ albums (Beer SQ was the album that made me stop buying that garbage series) or the DQ Brass series stuff. Piano and orchestra all the way.