Trades are not possible. You can sell your players, and buy other players on the transfer list.
It is theoretically possible to buy a player from team A, and have team A buy a player you are selling. But you shouldn't go above the market. And if the market value of a player is $XX,000; then it means that there will be several teams willing to pay $XX,000 for your player. If Team A pays $XX,000 x N for your player, it could be considered cheating, and if someone notices that you paid a more reasonable price for a player sold by Team A - it is going to look like Team A is transferring money to your team.
So best advice is not to think in terms of trades. Think of selling your players to raise some cash, and buying other players to improve your team as two independent transactions.