In general terms.
- When your player reaches 5 stars, the mangers a level under can't bid on him.
- When your player reaches 6 stars, the mangers at your level cant bid on him.
- One skillpoint from next star opens for negotiations
Reaching next star reduces the ability to sell him.
The best way is keeping him just before next star because then the level just under sees a player just before the 6 star ergo very attractive for them
Attractive for buyers, keep just before next star
(34-39-44-49 ... 99-104.... etc.)