Well, last season I bought a player who was 22, uncapped, with 15+ IS, 15+ ID, 8 RB with good secondaries and only a salary of $39K. Now, your trainee if he were ever to get to 15+ IS and 15+ ID with his rebounding at 16 would result in a salary of $110+K. So, is his rebounding really worth the extra $70K in salary? In my opinion, no. And, accordingly, I think that your player sold for too high a price. Your excessive training of rebounding hindered any decent development that player might have had.
But congratulations on getting a buyer.