That is true, but the more valuable a guy is, the more harder it is to do.
i.e you might be able to do it for guys that are worth like 10-50k or so but if the guy is worth 100k, it gets harder to pull off as theres more guys looking at those kind of skills. Also, the agents transfer fee tax eats into your profits as well so at the end of the day, each transaction does not yield that much of a return baring in mind that you also got to pay the salary of the player as well and there is risk that you can't sell the guy at the price you want to sell him for.