Welcome! :)
The higher the trainer level is, the faster your players improve.
Only stamina and FT do not depend on trainer level.
To begin I would recommend to take an advanced with a fitness spec if possible to delay stamina drops.
I advise you to read this manual page:
http://www.buzzerbeater.com/community/rules.aspx?nav=Trai...If you have any questions, please feel free to ask.
I am surprised you put on sale your 18 yo all-star.
I think you should keep and train him.