90% of my photography gears were bought used from craigslist or ebay. You just need to be patient and sometimes act fast. I miss 2 great deals recently on craigslist.
One is a 5D mk1 with 2 lenses and 550ex flash for $750 (the owner said I was the first one to call and will not accept any other offer. But the next day, he said his wife sold it)
The other one is today ... I saw the posting and called right away (15 mins after the posting) But it's too late. $1300 for 50mm F/1.8, 70-300mm USM IS, 28-135 USM IS, Tamron 17-50mm F/2.8, Sigma 10-20mm, 420ex flash, 580ex flash, 30D and accessories, battery grip for 30D pelican case that can holds the stuff mentioned.
Anyway, you just need to check often on craigslist and ebay for good deals. And need to do your own research on how much a item worth.