I guess a better way of saying it is that there is a pool of people who have already followed, or are currently following, a series that they read, and they create works derived from that existing series to create alternative storylines, using characters that are from a series they don't originally own.

A doujinshi implies that the comic is based on a pre-existing manga, or derived from a pre-existing fandom. It alternatively means that the graphic comic/short/novel is not licensed to, or copyrighted by anyone else besides the original owner. There are a lot of groups here on DA that cater to original comics and mangas, but this group here is specifically geared for doujinshis which are comics derived from a pre-existing fandom.

I answered another member here a little over half a year ago, but I believe that the answer also applies to your question too: [link]

Selling material that includes characters that are not licensed by you, or created by you can give probable cause for someone to claim you've stolen the characters. Manga authors work countless hours developing their style and developing characters. Everything from their name, to their hair, to their eye color... all of it is carefully designed, and the character design is copyrighted to the character's owner. If someone were to take that character and put that character into situations that go against the original artist's idea, they could get sued for infringement of copyright. Of course it is a case by case basis. A lot of artists appreciate fan art because it gives them an ego boost and gathers a group following, which is necessary for an artist to make money and pay his or her rent. However, when their fans start selling their own spin-offs of the original artist's characters and making money off of stolen material, without giving any royalties at all to the original artist, that's when artists, publishers, and distributors will come after that cash pool.

The best advice I can give, if you wish to pirate your drawings and make money off the black market, is for you to sell your material using low-key mediums like making a journal and having people note you, or masking your doujinshi in with a flash sale of doujinshis you own. OR, you could contact the publishing house and work out a deal in which you pay royalties per copy sold. It involves a lot of paperwork, and a lot of cooperation, but it has been done before, so there's always the chance of them wanting in on any extra cash. Be prepared to be screwed over in their deal, though, since you came to them instead of them coming to you. When they demand a substantial royalty being over half of your earnings, you can't complain, because you have no bargaining leverage.

So you CAN use Ebay if you want, but if you get caught, you can potentially be sued for the earnings you've made plus punitive damages, which is a penalty fine on top of that amount that is meant to "teach you a lesson" and teach a lesson to everyone else who would consider selling doujinshis in a mainstream marketplace. Just know that if you don't own the licensed characters, there will always be a looming threat of a suit. Actually, 100% of the time there is a threat. You can literally sue over -anything-. It doesn't mean you'll necessarily win, but often times people will get sued by a company like Disney, and even if they have a good case and are in the right and could win the case, what happens is that Disney has enough money to keep stalling a trial more and more until your lawyer's fees bleed you dry, and you're forced to either settle or lose the case because you can't continue the mental and financial drains. When you mess with publishing houses, they also have a large amount of money, and an eager legal department that will be more than happy to poach you, to set an example.

Just know the risks. In a nutshell, yes, there are a lot of copyright issues. The gray lining that you can argue is that it's either a parody or a derivative work. It is an argument that has been long-withstanding and keeps people out of the jailhouse for making fan art, but remember, if a company wants to sue you, they can sue you at any time for any reason whatsoever, and selling doujinshis of their licensed characters without paying royalties puts you at a -greater risk- for a lawsuit than if you were to simply draw fan art or fan comics for free.