Transfers question

Having only followed the Premier league for a few years now, I'm a little confused about some comments regarding transfers and how things work in general. I've seen in a few places that a team "will have trouble keeping" a certain player. I don't understand this comment and was hoping that some of you can enlighten me. Why can't the team simply refuse to sell the player? What makes it difficult for teams to maintain certain players?

 

It's basically evolved Contract law and pretty much the same for all professional sport, since the reserve clause was struck down.
And a lot of it started with a baseball player, Curt Flood sacrificing his career for it.

See http://en.wikipedia.org/wiki/Reserve_clause
and
http://en.wikipedia.org/wiki/Free_agent
which describes provisions of the Bosman and webster rulings:

Essentially it means that 'Indentured Servitude' is no longer permitted in sport.

 

I can understand at the end of a player's contract, but it doesn't make sense to me when talking about the January transfer window. I'm pretty sure that no contracts end in January. Maybe they're referring to a player whose contract is up at the end of the season, in which case it would be better for the team to sell the player than risk losing the player and getting nothing in return.