Black and White, Yin and Yang, Light and Dark.
HGSS and BW2 sure come in mind. (Like Jesus Lord, you can almost never get a copy for any of them for less than 80).Yea it's horrible. And a pain getting ahold of some games. They overcharge way too much.
I am so glad I'm one of the few people that bought both BW2 when they were still being sold (although admittedly I nearly couldn't cause I was only like 10-12 at the time and didn't have money on my own)