When Canon released the 24-105mm f/4L IS lens, many photographers jumped on the band-wagon and dumped their trusty 24-70L lenses in favor of the new kid on the block.
This caused a momentary, but significant, dip in the price of used 24-70L lenses due to supply and demand economics. I purchased my 24-70L at that time on eBay from a professional photographer who offered a two week trial. The purchase price at that time was less than $850 (U.S. Dollars) which was pretty much the going price of the lens at that time. I think new ones were selling for a bit over a thousand bucks.
My 24-70L lens functions perfectly and I absolutely love it. Although I had no warranty, I never needed to send it in for service and got my lens at an exceptionally good price. In fact, I could make money on it now if I desired to sell it.
OTOH - I purchased a used 17-40L which was slightly off focus. It was really not evident at first and the seller may not have noticed it, but I did! Unfortunately, not until I had owned it for a while. Since it was not covered under warranty, the trip to the Service Center cost about $250 (I don't remember the exact price). That put my bottom-line price of that lens above what it was selling for used. But, since the "L" glass has been going up in price, I could break even selling it now!
I guess that the moral of this tale is that if you get a good lens, you have saved a few bucks but, it you need to send a non-warranty covered lens in for service; the cost of that service will make the purchase a bad deal!
If you can find a CANON REFURBISHED 24-70L, that would be the way to go. However, I have looked at both the Canon site and Adorama (which often sells Canon refurbished gear) and there were no refurb 24-70L lenses available.