If you are the 'selling' club the only times it's viable to accept a co-ownership are (imo anyway)1. The player in question is not in your first team plans and isn't a hot prospect, so taking a bit of money now, whilst shrinking the wage bill is a decent deal, with the potential to either:a. get some further money later down the line if the buying club takes a liking to him.b. get the player back after 1-2 years and still have the option of selling him on, and who knows, superior training/coaching at a 'bigger' club may have improved him into a player that you feel is worthy of a first team spot. 2. The player insists he wants to leave but you want to retain him. He is rejecting all contract offers and morale is low. A co-ownership with the potential for you to regain full ownership in 1-2 years (by this time you may have won the league/got better finances/got a better reputation) when he may be willing to work with you again is better than losing him completely.