> If I'm the single developer and seller of this app
In that case you are also the responsible manager or product owner.
> Should the person selling or marketing the app be held responsible instead, even if they aren't technical?
Of course. The person who takes the customers money is responsible for delivering the result and any warranty.
> Why are the developers who didn't care enough to double-check their code free of responsibility?
They are not the product owners. They don't decide what is the correct way the product works. Maybe they created the bugs because they implemented the specification exactly as written?
> They don't decide what is the correct way the product works.
Of course we are, we're the ones writing it.
> Maybe they created the bugs because they implemented the specification exactly as written?
If your argument is "maybe they were told to write the bug in", I don't know what to tell you. If I were told to write a life-destroying bug into the software I worked on, I'd quit, because I don't want that on my conscience.
In that case you are also the responsible manager or product owner.
> Should the person selling or marketing the app be held responsible instead, even if they aren't technical?
Of course. The person who takes the customers money is responsible for delivering the result and any warranty.
> Why are the developers who didn't care enough to double-check their code free of responsibility?
They are not the product owners. They don't decide what is the correct way the product works. Maybe they created the bugs because they implemented the specification exactly as written?