Brian makes good points...
However, it is possible the saw is worth spending some $ on. Pulling the muffler allows you to see the exhaust side of the piston and intake side of the cylinder. Don't buy it unless it is clean looking, no obvious marks or scratches. Check the compression, either with a gauge or by pulling the starter. Make sure the carb can be adjusted properly. Make sure that small adjustments to the high needle affect the speed...and that the saw idles down fairly quickly, and idles smoothly. You want no air leaks. Check the plug color before doing adjustments, it should be medium brown.
I sold an old one for $225, bar and new chain included, after opening up the muffler, which produced at least 20% faster cutting times. I offered a satisfaction guarantee, and never heard back, so it must still be running.