To echo...
-Buy what you like. Remember, alot of it is just plain artwork, so value is subjective in many cases for basically the same thing.
-If buying new, stick to the authorized CRK dealer list (yes, there are even authorized eBay CRK dealers, so check first).
-The Exchange is a good place, but be sure to read the rules, especially about protecting yourself from scams.
-If you want the good deals on the Exchange, click the "Watch" button in the upper right and choose "Email notifications" for new threads. You'll have as good a chance at the actual good deals as the snipers and flippers have. The good deals and rare items go within minutes sometimes.
-Know your knife values; Sebenzas can run 2, 3, ??? times MSRP depending on whats available/popular. Don't be in a hurry and be the guy paying an extra 10% over MSRP on a used knife available everywhere new.
For goodness sake, have fun while doing it!