I just picked up my first DSLR yesterday and I can't wait to take it out this weekend... So today I'm looking to pick up a new 50mm f/1.8 ii canon lens and a used Canon EF 17-40mm f/4L lens. My question is, what are your thoughts on purchasing used lens, and what to look for when purchasing used lens? I saw these for sale http://sandiego.craigslist.org/pho/407930619.html and I'm thinking of buying it but I wouldn't know what the first thing to look for when purchasing used lens. Thanks.