I don't know how other sites do it, but Sporting News (TSN) is really slick. Players automatically rise and fall in value based on demand amongst those playing the game. You get a certain number of trades per week, you've got positions to fill and a budget/cap to work within. It's all automated too, based on the actual NBA season/rosters and schedule. So, if Kidd only has 3 games in the next 7 days, you might trade him for Marbury who could have 5.
In the past, I've thought about building an engine to do it, but there'd be a fine art to perfecting player values and automating the input of all player stats (you get points based on all of the stats combined in TSN).
It'd be cool to be able to buy a good engine and just switch the schedule for that of the NBL along with the players and stats.