Daytrading is buying and selling players for the sole purpose of making money off of them, with no intent to have them help the team on the court. It was rampant in season 3 and has since toned down a bit, but it is a way that a lot of very strong teams have become as strong as they are. It is the reason there is a time-related % on transfer fees. Some managers think it adds greatly to the game, others feel that it detracts from the game. It isn't easy to be successful at (reportedly) and it isn't expressly against any rules.
Once I scored a basket that still makes me laugh.