In this post I'm going to be talking about how rugby union clubs like Toulon buy players.  I will describe the inputs and processes and then obviously the output is the player. 
Unlike other sports like Soccer and American Football, there is no set time that transfers can happen only in this set time. 
What usually happens is that when a player's contract at a club is coming to an end, other teams will put in offers to that player to try and get them to their club.  If the team that the player is currently at want to keep said player they will also give contract offers to the player.  The player will look over these offers and will choose the one he wants.  Which one they choose can be for a variety of different reasons, whether it be money, importance in the team's set up or even the place where the club is based.
Once they choose their favourite contract they will accept the offer and then move to the other team when their contract with their current team runs at the end of the current season.
The reason Toulon get so many world-class players is because they have a perfect mixture of all the things players look for in a team.  They can give players amazing salaries, Toulon is a beautiful city and the players they sign will always have a good position on the team.