Making deals with other managers is a huge part of the game, not only to help bolster your own squad, but also to ensure plenty of banter and keep the game thriving. Naturally business is busiest during the off-season and early season, and then settles down as managers feel their teams are ready to take on the rest of Europe!
Please see the below links relating to transfers in PBeMFF:
Download this handy spreadsheet (uses OpenOffice, a free software which is also compatible with Excel) and type your player's stats over the top of the default player to generate a *recommended* value for the player. Don't always expect to get the maximum though!