I would expect to pay $1300 - $1500 for a lightly used 5D3 in excellent condition. Since these were widely used by pros, you can find some with heavy use and/or cosmetic defects for less. It’s an excellent workhorse, but I’d be VERY skeptical of one that sells for $500 (better make sure you have return priveliges and it’s a reputable seller). There have been a couple for sale on UHH during the last month or two.
Thanks, yeah, we weren't looking for a pretty one. Actually, I have found several in the 700-900 range.