Are there different auction house categories by championship or something? I've just noticed that the general standard of players being sold is much higher than when I was in championship 1. My initial plan to develop some players through training, to then sell seems to be pointless as I'm about to go up 2 championships and if I can't sell players to users in lower leagues there is little point having them, I'd be better off unloading them now.

So any insight to the auction market would be appreciated. How many markets are there for example?

Yes the higher your championship level the more access you have to higher quality players also when training you can push them higher aswell

Is it higher access or a different market subsection will championship 1 users see my 15 star player if I'm in championship 5?

This is a question I'd like to see answered too. From my experience it seems you are lumped in auction houses only with those at the same tier as you. Some confirmation of this would be good

You only are allowed to buy players sold by those teams at your level.