Firstly, yes they show up. I've sold 6* players before including a Donkey of a DMC.

As Sully says, the issue for you is how attractive they are to others i.e. how sellable they are. If they have poor ratings/stats then that will go against them with managers that actually look. Additionally you don't say where they are in the Q band. If they have only just made the 6* then they're low Q - and a possible solution to this is, as Sully says, improving their appeal by collecting sp to raise their Q for neg or if thats not possible, at least raise their Q bit by bit and keep listing them. As long as you don't need space these sellable issues are less in the new Season.