I found an easier way while going for the 'Law of the market' solution:
Summary: Create two teams (12 players and 0 players) and sell one player to the empty team for 500000.Update: The buying team needs to be on a separate account now! Thanks to Webhead2099 for pointing this out!
- Create a team (selling team) with at least 12 players.
- Play a match (not sure if you need to, I already played several solo matches with that team).
- Now create your own multiplayer league, also create a competition (otherwise your teams can't join)
- Leave the competition you played the match in with the selling team, and join your MP league.
- Create a second team (buying team), but do not buy any players, back out of that menu.
- Join your MP league with your buying team.
- Edit your MP league, edit the competition and delete the competition (not the league!)
- Now you can open the market in this league.
- Edit your selling team, choose a player to sell, put on the market, set selling price to 500000.
- Edit your buying teams roster, buy a player,
to Transfers, bid on the player.
- Back out of the team,
to My Transfers, accept bid.
Achievement should now unlock. If not, check your buying teams page and please comment.
You can also use this method to get gold for the Coliseum achievement (just up the selling price even more or sell a second player).