It's easy to buy coins, but expensive. So I can't imagine many people to illegally acquire Aguero. To buy enough coins to get Aguero, you'd have to pay almost £60. £185 for Ronaldo.
How it's done? Let's say you want to buy 20 000 coins. You put up two shitty players for sale and set the buy now-price to 10 000 and the starting price to something irregular like 6 750. When you pay for the coins, you add a comment where it says what players you've put up for sale and what starting price you set. They will buy your players.