Negotiations is the way to go, i buy 4 or 5 18yr olds every season, keep training them and then replace them every 2 or 3 years. i always train them to 79/99 when they arent in the side and then sell them on negotiations.
You also only need a squad of 18 at most so the remaining 10 spaces are reserved for buying players for 1 token at 57 - 59 or 77 - 79, train them and then selling them at negotiations. big profits every season mean you can easily do this.