It wasn’t that difficult because I bought the console on day 1 and during the first few years I was buying almost everything.
When LRG launched Breach & Clear I was there buying it as well. I bought every LRG game when they released them (only missed 2 or 3 and then bought them on ebay, including Super Meat Boy).
I was also lucky to find another Vita collector here in Brazil selling his collection so I bought a lot of sealed games for a good price.
But there are a few expensive games already that can cost you $200, $300 and maybe even a bit more.