I use CL exclusively. You need to decide what you're willing to take. Personally, I only buy for 70% or less of the current new item price. I've had good luck selling at 75-80% of new. Include good pics in your ad, product -type pics ('seamless' white background, a piece of paper works just crop tight). Ask for a phone number in the reply.
+1....on CL. I recently sold my 50mm f1.4. I posted at 3:30PM and the lens is sold by 6:30PM same day. The lens sold at $290 - current price tag for NEW is $360.
As Neuro mentioned....you need to know current market price and don't expect to get 90% of new. Under html, post 5-6 clear decent size pictures - make your decripstion short and clear. The main things are PICTURES & SELLING PRICE.