You need to be part of a league. There is no transfer market in COL to my knowledge to avoid farming if I m not mistaking. But if you are part of a league there is a little window between season where the admi open the market and you can trade players. I never did it either so i can not inform you further
Best way to get the market achievements is create an alt account. Play via the AI until you get a legend (knocking out that achievement) which can be done in about 10-15 games if you feed only one player TDs, and then create a league with just the team your legend is on, and one team from your alt account. Instead of buying any players on your alt account, just buy the Legend from your primary team. Boom, achievement unlocked for selling a Legend.
You have to do this, as the Market is a dumb idea that serves no purpose. But at least it isn't Real Time Mode.