The Poly 61 was my first "real" synth, so I've got a soft spot for it. You'll never get the guy down to $125, as one poster suggested. In the post-eBay age, no one is giving away synths for that price anymore. Even a non-working Poly 61 in halfway decent shape will sell for $200 on eBay. At least the one you're considering is mostly functional.
If you buy this synth, you should plan on spending a couple of hundred bucks to get get it fully inspected, get the battery changed, the key contacts cleaned, and to possibly replace other faulty components. Once you're done, though, you'll have a very nice, working vintage synth.
Just make sure to figure repair costs into your buying equation.