http://www.operationsports.com/forum...reer-mode.html
The most complicated thing is probably the transfer market but even that is essentially just a simple trade system. You make a bid to a team for a player and they'll either agree to your terms or tell you they want more money. Either way once you settle on a price, from there you negotiate with the player concerning their wage and role at your club.
The same thing applies to you selling your players. You can either put them up for sale and see who bites or if they're performing well enough other clubs will take notice and make you offers. You can either agree to the amount of money they want to give you or ask for more if you think you can get more. For the most part it's common sense. If your player is worth 2 mil and you're getting offers from 5 different clubs for 8 mil on average, that's a pretty good indication that he's in high demand and you can negotiate for a higher selling price. Once he's sold you get a % of his fee which depends on what you set your board strictness to at the beginning of your career (60, 75, or 85% if I remember correctly).