a brand new 290 is around $500 here in massachusetts, and we have plenty of dealers competing against each other.
im going to be the lone dissenter and tell you to skip it, in my opinion, there are better used saws for the money if you are in it for the long run. anything that goes wrong with a farmboss tends to cost more than the saw is worth, making them more or less disposable.
the first saw i bought was a used and "freshly rebuilt" ms290 farmboss for about the same price and in what looked like great shape. 1 hour into the job i bought it for, the saw was down on power, in hindsight, thanks to a leak at the cylinder base that made it run lean, thanks to the permatex job that the last guy did. the farmboss does NOT use a base gasket, only a bead of silicone.
further, the ms290 is a clamshell design, meaning that half the crank bearings are held by journals that are cast into the jug. see the half rounds below? that is a tolerance fit and it creates a few issues. the least of which is that the saw isnt really able to be modded, the bigger issue is that if the base warps or gets knicked, you cant resurface it because the bearing wont let the jug seat any deeper.
because of this, cylinders are very expensive and only available from stihl in my experience. i was quoted over $300 for a new jug. with so many homeowners tweaking the screws, there is a lot more demand for good used jugs than supply.
also, you can see here that the crankcase is made of molded plastic, and its EXTREMELY inconvenient to work on because you really cant strip it down much more than this, everything is a really tight fit and difficult to get apart.
of the 7 saws ive torn down, NOTHING has been even close to as aggravating as the farmboss. you have to strip every last piece of that saw to get the jug off, and the chainbrake is ridiculously over complicated, made in germany alright. only a nazi could want to make a human suffer through reassembling that thing IMO.
the only other stihl ive worked on is the 041AV so i cant really speak for stihl overall, but i would never buy another clamshell, ever again. all the husky, dolmar and jonsered saws i have tore down were a joy compared to the MS290. i sincerely recommend you seeing if you can find a jonsered CS21xx series saw. 5 very hard years and all ive changed is the plug. should be in the same price range.