Players loss some percentages only if their team promoted or raised one level. Actually, the players do not loss 1 star. They are just shown 1 star less because players of opponents around them are stronger after promotion.
Usually, these players will improve more quickly on training after they are shown 1 star less. You should consider this before selling them.
I agree that scout players are too expensive in terms of tokens. So I just recently got my first scout player after playing for 3 years. I always forget them and focus on competing with players that I am capable to obtain.