Okay we got buying new manga down but what if want to buy used, here are some tips for that. First why even buy used? If there is an old series that now seems interesting used maybe the only way to get it. Also used is a great way to pick up stuff for cheap.
Brick & Mortar
One place to try is your local used book store. Much like going to conventions to buy this can be a huge crap shoot. Some used book stores have a great section of graphic novels with manga for good prices, others well not so much. The nice thing about the used book store is you can take books you have laying around in for credit to use on picking up stuff you want. Used books stores sometimes end up in weird places so it might be worth Googling it to see if any are in your area.
Then we go back to the local comic book store. If you are familiar with your local shop then you will know if they carry used books or not. While back issues of single comics are common, used volumes of graphic novels and manga not so much. It can't hurt to ask though.
Another shot in the dark is a book sale. These are local events held by generally by libraries (but other places also) selling off a huge mass of used books for super cheap. I've found some very interesting books at sales like this but the manga kind be kind of iffy. A lot of the time the people running the book sale don't know where to put manga so you may have to look in children's and sci-fi sections.
Another brick and mortar store I would highly recommend is Book Off. Book Off is a Japanese chain of used book/video game/DVD etc. stores. Currently there is one store in New York City, five out in California, and two in Hawaii. They will often have a good selection of manga in English, anime DVDs, and art books. Most of the manga here runs about $5 dollars or less. When I was visiting friends out in LA I think we visited almost all of the stores in the area. I brought back more stuff from Book Off then I did from Anime Expo that year. I also make sure to stop in the New York store every time I'm in the city. Now, what if you don't live close to one? Well you get someone else to look for you, which I'll get to more in a another post.