Used Kindles might be preferable in theory, but there you have firmware of no return issues, don't you?
Anyway, I just mean that if you bought a used H2O for € 50, for example (not sure what you mean by extremely cheap), imo you'd be getting an excellent device. I'm sure the same applies to a roughly equivalent Kindle. The Libra is € 180.
Of course if I'd already made up my mind and budgeted ~200 for the purpose then whatever.
For KOReader support definitely the Libra, but one question potentially worth asking is whether PocketBook has better build quality?