Hey Ashley,
That's because Greenman, buys them all. He gets up at the crack of dawn to check the classifieds and has the ads pulled before any of us wake up. LOL!
The reality is probably the case that Greenman made, and that is that most adults are kept for future breeding projects and owners of such giants are not looking to unload their adults, but rather the off-springs of those giants instead.
However, I have managed to put my entire adult collection together from the use of the classifieds and have been pretty successful at it; so it's not impossible. You just almost have to be in the right place at the right time, so to speak.
-Angel 
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"Until we lose our self, there is no way of finding our self."
-Henry Miller.