Yes, Play N Trades (depending on ownership) carry EVERYTHING. A store near me had an R Zone and a Game.com, they've also got CD-i games on the shelf. So yes, the carry stuff older than Gamestop, and yes, they carry Game Boy Color games.
Play N Trade stores are all independently owned and operated, so pricing will vary greatly from store to store. If the owner knows what games are worth, you'll find prices to be around eBay, but if the owner is just in it for the money (and has no idea what stuff is worth) expect everything to be about 55% below eBay price (the POS system underprices a lot of stuff).