There's a small-trader exchange called Priceminister based in France that offers sellers and traders an on-line "shop-window",
www.priceminister.com. There's a lot of French-translated manga published in Europe that doesn't get an English licence and quite a bit of that can be found second-hand on this site. Look for "livres et BD" -- bande-dessinne means comic strips, sorta and that includes manga. There's also original Japanese tankos and such offered occasionally. If your French is better than your Japanese this is a useful facility for obtaining some manga titles at reasonable prices.
The website's in French with no decent English-language version. They only offer delivery to European addresses but that includes the UK where I am. It's a bit like eBay, you deal directly with the sellers ("vendeurs") but you can pay via credit card through the central Priceminister site. It's not an auction site, prices are fixed but you can haggle if you think the sellers are asking too much for a given item. You can also register a wish ("souhait") for a particular item with a set upper price limit and if the item is listed at or below that price you will be emailed to alert you to have a look.