These are the sellers I as a manga/light novel reader personally loved buying from.
cheapmanga.com sells remainder books (Remainder books are essentially brand new books they just have a sharpie mark on the top or bottom to indicate overstock). This is a good place to get manga/light novels for a cheaper price. Their selection is quite limited though but if there's something you're interested in on the site it's a good place to buy from. They're generally pretty friendly if you want to send them a message on your order.
The best place to buy Manga/Light Novels in bulk. Their products are always priced at 75% MSRP and very often do sales & coupons that bring that price lower. There are a few catches though. One is that because the customer base is so large it can be hard to get the books you want, my suggestion is to separate in-stock orders and out of stock orders so that your order doesn't take months to arrive (they will sometimes hold your whole order if an item in the order is out of stock). As of recently they've included a shipping insurance service called Route that handles all their shipping damage/lost order claims. I highly advise buying the insurance as I've heard complaints about shipping as of recent.
A good seller for manga/light novels on ebay. They sell many different sets of manga/lns. They generally sell their products at ~50% MRSP with the only catch being the books are remainder books, but all that means is there is a small sharpie mark on the top or bottom of the book. By no means does this hinder enjoyment of the books.
This is a reddit community dedicated to buying/selling used/new manga. The advantage of buying through here is you don't have to pay ebay fees or tax fees, only a 2.9% paypal fee. The buying/selling procedure is very safe as long as it's followed (Just read all the rules). I'd advise to use an app called alerts for reddit if you'd like to get into buying from this subreddit (you can set it up to give notifications when a new post on the subreddit is made).
The reason this is an honorable mention is that because the community is so big nowadays it can be hard to get a good deal, nonetheless they do pop up on here so it's worth mentioning.