A very nice 036 will go for $250-$350 here. You will hear about guys who get screaming deals, and it does happen, just not that often.
I would look at the external condition first. Has it been abused? Lots of paint loss, cracks in the plastic, etc. Does it look like they dragged it behind their truck? I have saws that I have used for years, so I know what a saw looks like that has mileage on it, but has been cared for.
Personally, I'd stay away from professionally used saws, as these are usually sold when they are close to dead. Ask the seller about the saw's history? What kind of mix did he use? Did he drain the saw when not in use? What kind of bar oil does he like? If he has some fair knowledge about saws, that's a good sign.
I would rather pay a little more for a saw that I have more confidence in, than get a total steal on a questionable saw. Since I have only mediocre tech skills and limited time to spend on wrenching (unfortunately), I will usually stay away from saws that need extensive work, even if they are super cheap.
As mentioned do some searching here on saw buying tips, and check out craigslist a few times a week to get a read on your local market.
Good luck, and post pics when you find a saw.