Set his valuation to something outlandish, way above what he's worth.
Then, when the next enquiry comes in, say you'll only sell him for the right price.
Sit back and wait... Eventually a bid will come in at the valuation you've set, which the board thinks is too good to refuse.
In my current game, I recently sold Romeo Castelen (worth 7 mill) to Aston Villa for 15 million euro. Suckas! He's not even my star player.