I've actually been in rooms when it's done . I watched an agent (it was years ago and not the fella I know well) get telephoned by one club for a centre half then ring another clubs manager and say ''are you still looking to get rid of player x ?" . Then in the space of 20mins say "they don't seem convinced by him you'll have to be flexible " and "they don't really want him to leave it'll cost you" . These weren't top clubs but he finished the calls after about 20mins and said "that's how you earn £x" . Nowadays obviously you can only work for one club in a deal but they will just introduce another 3rd party agent they know into the deal.
The amount of agents used in a deal is fascinating as if everton want to buy Player A , they might not know his agent too well but they know Agent1 who has a great relationship with the player they might approach him who will then go to the players agent 2 and say you sit back I might have a move and a nice few quid for you or you can do it but the club don't like you . So agent 1 gets paid by everton but Stoke who are selling might use Agent3 for their deals so Agent1 approaches him and in the earn everybody earns out of it. Mad really but they are now well ingrained in the game .
Or when a club want to sell a player they might approach multiple agents and say we want £6m for him but if you can earn more you get a substantial percentage of that extra fee.
Some agents hardly handle players any more they just work for clubs buying and selling, it is fascinating . I'm told repeatedly anyone can be an agent but unless your lucky enough to have a top top player on your books you need to have a phone with the managers , players, agents & CEO's numbers on or you'll never really 'make it'.
I'm off out so writing that in a hurry , I'd always try and pass on a few bits if people were interested though