You need a Gold membership to view other users want ads. Gold level users can see your ads and can respond to your ad, but you dont have a membership level high enough to view those same ads. If you dont buy a basic or gold membership I suggest adding your email address to your ad so gold members have a way to reach back to you. Without at least a basic membership you wont be able to receive Private Messages.
Hope this helps! I had the same question about 7-8 months ago.
Your results may vary. But I found the gold membership very worthwhile as I have managed to save way more than the cost of the fee on getting great deals on selling my unwanted blades and doing trades. The WTB forum is also good if your looking to unload a specific knife as you may find targeted buyers and may sell your own knife quicker.
Jeff