Fredmiranda.com Buy & Sell has a huge Canon marketplace. You will need to register to search Buy & Sell. You will need to pay a small fee to upload pictures.
Canon has very good resale value. You just have to price it correctly, show good pics, and be smart about who you are dealing with. Luminous is a great marketplace too - a lot of pros and people with established history.
Looks like about $1,000 for the 24-70, to $1,250 for the 70-200. The 70-200 has a II version, the 24-70 has a new version coming out soon, but it is $2,300.
You can also pick up the Nikon version of the same lenses there.
You will sell at the correct market price without problem.
You do have to do your best to "prove" who you are, and show some kind of internet history - web site, auction site sales, etc.
Most people will want to work with PayPal.com, or ChaseQuickpay which is free.
I had a Nikon D800E on order, but decided not to switch now as I have far too many Canon lenses. Did not want to mess with it during the summer - maybe in the fall. But I have 7 L lenses, plus 4 non-L.
I am actually looking forward to the Canon T4i this summer in the $850 range, mostly for video.
Good luck!
Michael