There are plenty of great used lenses for your EOS camera here on eBay, but you need to be smart about spending your money, not everything is a great deal. So I've put together a list of some of the more common EOS lenses along with their price new (for US versions, from Adorama.com) and typical used price. These are current as of now (August, 2007). Prices don't include shipping, and some used lenses may come with hoods or filters not included in the new price (only Canon L series lenses include the hood). Also, Canon has at least two rebate programs/year typically that can reduce the net cost of a new lens. Also, remember that a new lens comes with a warranty, Canon USA warranties are NOT transferrable, so that used lens claiming to have a warranty better come with a recently dated sales receipt (with no name on it) or have a third-party warranty such as a Mack warranty (which is transferrable to a new owner for $25).

17-40mm f/4L - new $629, used $550-575

18-55mm (kit lens) (NOT USM) - this isn't sold separately, only with a new camera. Typical prices are $50-70 used.

18-55mm (USM - only lenses that say USM on the front ring are USM lenses) new $139, used ~$100.

28-105mm Mk II f/3.5-4.5 USM - new $230, used $150-175

28-135mm IS - new $409, used $350-375

35mm f/2 - new $229, used $180-200

50mm f/1.8 (Mk II version) new $75, used $55-65

70-200mm f/4L (non IS) new $579, used $475-525

70-200mm f/2.8L (non IS) new $1140, used $950-1050

75-300mm III (non-USM) new $189, used $140-170

75-300mm III (USM) new $145, used $100-125

100-300mm (USM) new $279, used $210-240

Without a doubt, the lens I see people spending too much for time after time is the Canon 75-300mm (both the non USM and USM models. There are folks asking over $200 for the non-USM lens, and over $225 for the USM one, don't get "taken", either buy it new and get a warranty, or shop for a reasonably priced one. As I write this, I have 6 of the USM model sitting on the shelf to be sold, I hope when I get around to getting them listed people have done their homework.