Something to consider: pick up a used lens from a rental house. Obviously, you should get to know their practices on when to sell gear, but you'll most likely have a return policy in place, decent testing before the sale to know what the optics are like, product photos to know the condition of the item, and a sense that once every 1-2 weeks they laid hands on that particular lens. I've bought two cameras (admittedly 1Dx which has a rounded-up odometer), a 300/4IS, and a Profoto D1 Air kit from LensAuthority (sales arm of LensRentals.com) and couldn't be happier; I'm saving up my pennies for another bang-up year of photographic purchases, mostly from them.