The closer second hand lenses are to "mint condition," the closer the price will be to a new price. If the item includes all original accessories the price is boosted, and even the inclusion of the original box will render the item more attractiive and therefore more costly. There should always be a marked drop in price, however, just as a brand new car depreciates the moment you drive it off the dealer lot.
If you are searching ebay, use the "advanced search" option to see how much these lenses have sold for in recent weeks. I apologize if I'm repeating something you already know, but you need to tick the "completed listings only" box to get access to these finished transactions.
It's always comforting to get a new item with a full warranty, of course, but most camera lenses from major manufacturers don't have all that much that can go wrong with them. The VR/IS lenses are an exception, of course, and I would advise people to be careful about buying these models second hand without being sure all is in order mechanically.
You might ask the seller(s) whether the lens originally had a warranty valid in the country where you live, as with some brands, at least, the distributors will not even do repairs off of warranty on grey market items.
You also should know in advance what the return policy is for your item. If the seller won't agree to a short period during which you can return the item, don't deal with him or her. Then when you get the lens you should try it out vigorously right away to make sure it meets your expectations.
The truth is that when you get second hand items, particularly from non-dealer sources, one of the reasons you pay less is that there is an element of risk involved.
Again, I'm sorry if I am belaboring obvious points here, I don't mean to preach. But I have bought a lot of second-hand gear on ebay, and I've learned to be circumspect. But I've gotten some terrific bargains in the process.
