Here's the thing. If you sell things, you lose control of that item the moment the other person has paid for it and it is in their hands. You can't control what someone who purchases something does with it, nor should you be able to. Either you deal with that fact or you don't sell shit at all.

I do costume commissions occasionally for some extra money. If someone buys something from me and decides to alter it slightly in whatever way, I have no right to tell them they can't do that because the costume belongs to them now. I would have a right to complain if they were passing it off as their own work or using it to make a profit, but if they loan it to a friend for a con or a photoshoot or even decide to resell it eventually, that's their right.

It's the same with doujinshi. Scanning and translating it and charging money for the scans or translations would be wrong. But doing it for free? That's on the same level as loaning a costume to a friend. No harm, no foul.