To be more specific: is there a console command which revert bought horses to a state where you can repurchase them?

I'm curious to what your issue is for asking for this is? Do you have a bunch of horses following you and you don't want to kill 'em or what?

http://skyrim.nexusmods.com/mods/13530 let you unmark them so that they're not yours anymore. Don't think it will return to the stable so that you can buy it once more tough.

The reason why I desperately need to revert them is that I somehow messed up my horses via trying out some mods (dragon cart, convenient horses, more saddles) and I ended up wth no saddles showing up on them.
The ones that I have not purchased yet and are in the stables have saddles, but the ones I own are bareback.
I have tried disabling all the mods affecting the horses, also disable/enable on them via console but to no avail, they are still bareback.

You ended up with invisible saddles because of compatibility issues between horse armors from CH and the saddles added by More Saddles, is my guess.
You can try disabling horse armors in the CH config menu and after that reload the horse model (disable-enable) or ask More Saddles author to make a compatibility patch for CH horse armors. Alek has provided a PDF tutorial and it's really, really easy.