I prefer the four star players, especially if I want to add a role or SA.. they train faster than the five star. On my server there's almost always bid wars on good players.

Sometimes it is better to get a player from a team if their Attributes are set up the way you prefer. (and if they are super high price he's prolly a good trainer too)

Generated players are almost perfectly even. This is why sometimes people complain, oh I got this five star and he is getting crud rating. Well, he needs to be trained to work with your team, and if he is already five star it might take awhile!

The best time to buy is when your server is slowest.