Buying used is more of a risk, but with a few simple checks can have some great rewards. It can take some work, and will take more time, but you can get a saw for cheap.
On ebay, you need to be prepared to bid on many saws over time to get one at the price you want. It is harder to get a deal on a smaller saw there though. The bigger saw market can be more finicky, and deals are more likely. Buy from a guy who has moved some saws with a good rating. I got my 066 there (there aren't many of them on the prairie) for $400, not pretty, but mechanically sound.
Garage sales can be good or bad. I bought my Super XL for $20. Chain would have cut better backwards, but the rest of the saw was remarkably nice. Scan the paper, I've seen some fairly decent deals, and often the seller is willing to negotiate. Chainsaws don't sell too well at garage sales in most towns. The point there is to get rid of them.
Pawn shops? If you get one with a clue (one that pops the muffler before buying), you can do okay, but it takes some time. Try to stick with independent shops, we've had a bunch of pawn chains pop up over the past few years. They tend not to deal unless the thing is growing cobwebs, and have "parameters" unless the thing has really been camping out. An independent guy is going to be more cash flow sensitive more often. As Lakeside mentioned, this is a good time of year. Also 2-3 years after a disaster of some sort. We had a huge ice storm a few years ago, and the used market perked up a bit, and will probably be better than average for a few more years as homeowners get tired of tripping over a saw they will never use again. I was looking at a couple of nice little saws with "cobwebs" on them and had started to deal with the owners when there was a big ice storm in that area. But, the saws will be back, or ones just like them.
Patience, and a screwdriver (torx for Stihl), and willingness to find out how the compression should feel like.