If no one wants him even on a free, there's nothing you can do to sell him. What you can do is try to push him in a transfer for someone else. You can try to add him to a deal for a new player you want. If push comes to shove, you may search for players who may accept to come to your team that play in one with money to pay wages of the guy you want to offload and that hopefully you will be able to sell that one later. Let's assume you're Crystal Palace, you can go to Liverpool, look for a youngster that's not going to make first team there and is on low wages, you exchange them you get a young player on less than 25K that you can develop and maybe sell for some decent money in a year.