I sell as and when required. No right or wrong. At level 1, selling a player whose quality is 18 or 19 is more attractive to buyers than selling a 20. Psychological thing I guess.

Buying the the tricky bit. Some ppl avoid the early season rush and buy a 5 star player on the verge of the 6th star at the end of the season when there is no competition to buy. When they eventually lose a star when the new season comes, it wont be long for the player to gain skill points to get back to 5 stars.

Early season there will be loads of traffic in the market so u may end up losing more tokens than you can afford.

So it depends on your preference and patience.