Trades with Poison Pill contract require either cap space or trade exception. In a trade Sanders would count as $3 million contract for Bucks and $11 million for his new team. It means that Bucks can take back only up to $4,6 million in salary (150%+$100,000), but Kings have to send out at least $7,266,000 [($11,000,000-$100,000)/150%]
The difference as you see is $2,666,000 and a contract(-s) of at least sush a value has to be sent by Kings to a third team with cap space or trade exception.
That's why trades with such contracts are rare.
P.S. As always you can read it in CBA FAQ:
http://www.cbafaq.com/salarycap.htm#Q88.