100. How are teams able to trade players who are out of the league, like Dallas did with Keith Van Horn?
The short answer is: they can't. Under previous CBAs teams were allowed to sign-and-trade (see question number 89) any player to whom they had Bird rights. Bird rights don't automatically go away once the player is out of the league (see question number 32 for details), so many teams continue to hold Bird rights to players who have long since retired (for example, as of this writing in 2012, the Lakers still hold Bird rights to John Salley, who last played in 1999-2000).
Teams were able to take advantage of the Bird status of retired players when they needed to include extra salary in order to make a trade legal (see question number 81). By signing players out of retirement the teams were able to complete the trades, and since a sign-and-trade requires only the first year of the contract to be guaranteed, it was like they were trading an expiring contract (see question number 99). This kind of trade occurred twice during the 2007-08 season, when Dallas re-signed Keith Van Horn (who had been out of the league since 2006) and sent him to New Jersey as part of a package to acquire Jason Kidd, and the LA Lakers re-signed Aaron McKie and sent him to Memphis as part of a package to acquire Pau Gasol.
The league put a stop to this in the current CBA by restricting sign-and-trades to players who finished the prior season on the team's roster. A player can no longer be included in a sign-and-trade if he has been out of the league