I went to an independent used book store today and discovered a whole shelf of Japanese manga, half of which were Yaoi! I bought 3 at $2.95 each with tax for a total of $9.60. It's better than buying new ones from Kinokuniya, and I don't have to pay shipping and handling for online.
I didn't find the one I really wanted, Neko Samurai by Kyuusyuu Danzi, but i saw some of her other works, and I bought 2 of them, and a cute one shot with a tender seme in suits and glasses (my type) and the feminine and cheerful uke.
The bookstore also buys used manga, but they usually pay $0.50 each, and they charge $3 to buyers!
Might as well sell manga as cheap as $1-3 so you can keep as much profit as you can.
But it's useful if you have trouble selling manga, especially online, and you want to get rid of extra stuff.