Could be wrong about this but I think it depends on 2 things to why none buys them.
1.) you are asking too MUCH for them
Like lets say you have a 1 star player you shouldn't ask for a hand and/or a leg meaning like 5 million for him/her
What I do no matter the star is when I put them on the transfer list I ask for the first price that it gives me.
Also starting price is lower than the market value .
2.) I think this one is more important it depends on how many people are online at the time you have to sell your player.
like if you have 2 hours and lets say there is only 1 person online at that time then most likely you wont sell your player.
But if there is like 100,000 people on there at that time then more or less he/she will be sold because someone will see him/her and make sure to get that player to help their team.