yes only 99 79 59... rated players are set for negotiation that is the rule.
If you plan to sell a player through nego. then you shouldn't use them in matches or train them once they reach the quota. If they gain man of the match or the daily skill bonus they will have a 4% increase in a random skill that sometimes results in a 1% arquality increase, so be careful there.
Teach them a special ability or new position so they won't gain more skills through training.