If you’ve got some manga you don’t want anymore, you can always give it away, but you might want to try swapping it too. One list that I’ve used for a long time to swap manga is manga trade, but there are a couple new sites I’ve used recently that have also come in handy for trades.
Mangatude has you fill out a profile with a list of the items you’re willing to trade, along with your wish list. You can swap anime and other graphic novels there too. There’s a wish list search feature, so you can check to see if a title you’re offering is wanted by someone else. I’ve also used swaptree recently. Swaptree accommodates three way trades, so the number of items potentially available for you to swap for is much higher. I’ve traded manga and books around four times on both mangatude and swaptree. It always is a bit chancy when you’re sending stuff off into the mail to people you don’t know, but I haven’t had any problems at all.