I personally don't sell players often although right now I'm heading into the January transfer window with two of my purchases have not performed as I had hoped in sixteen months of being at the club. Toying with the idea of selling or seeing out the season with them.
One player, Kevin GroBkreutz he is okay but plays angry all too often and it seems is carded every match. Loses the ball very often (not technically gifted)
Another, Robert Snodgrass is the polar opposite, always nervous, a timid fellow who to me seems very overwhelmed. Come the big match I think he'd crumble. But stat wise and scout reports say (I don't use FM Genie) they seem very good for the three types of role I use wingers for within my systems.
I also had issues with Lloris wanting his wage more than doubled to ?170k a week after one season at the club, I told him no,he reacted badly so I bought Mandanda (release clause) and now just waiting to see if Lloris calms down.
I'm sort of nursing my way through this year really as my youth prospects are still a year to eighteen months away from being anywhere near ready, I have sold Michael Dawson and brought Steven Caulker through as rotation starter behind Kaboul.
I have a fist full of cash but resisted all but one major signing this year, Victor Ruiz (left CB).
So I'd have to say the best time to sell, or list players is the off season. No risk of a player spreading disharmonious vibes mid-season. Buy and make offer for player's at anytime imo.