That's a fair price for a nice 056 Mag I think. Check all the usual things, compression, rewind, sniff the gas tank, and check the oil pump. I've had two of those go bad on the 056's I've had. It'll be leaking out the adjuster screw head. A new pump is $35 or so on ebay. Make sure to the best of your ability that the cylinder is good. You don't see that many Mag cylinders on ebay, a few more Super's and a fair amount of standard cylinders. If it looks nice, and they'll let you pull it over, or even start it, go for it. It'll sound different than a newer, more current saw. It's a much slower saw, as far as RPM, so it may not sound as smooth as say an 046 would, so if it sounds rough, it's probably not as bad as you'd think. Mine isn't as pleasing as my other saws are to listen to, if you know what I mean. The 056 is a nice old saw. Heavier than hell, but a nice old saw. I love mine. Let us know how it goes.
Take care all,
Jeff