I wouldn't buy any Polaris 800 from 2002-2012 or so. Known to have issues to say the least. BC_Dan knows more about them then I do but. A lot depends on miles and what modifications have been made on those if he piped it I wouldn't go near it but that is me.
The big block 800 has issues with the crank pounding the crank journals out, enough that the crankcase can no longer support the crank. That often ends in the PTO end of the crank breaking off, or a break at the connecting rod. The issue was more common in the mountain sleds, the trail sleds fared a bit better. At any rate, all the big block 800's had that design.
Another failure point in the 2002 was the VES valves. The original valves were made of aluminum, and as there is pretty much no cooling around the valves, they would get hot and melt, dropping into the cylinder. Replacement valves were first made of titanium, then stainless. If the VES diaphragms are dark green instead of orange, it is not likely the valves have been replaced.
The 800 had a lot of torque (part of the problem!) and could be made to run really well. I added an engine plate and a couple of torque stops to help keep the engine from flexing on the two that I had. But the biggest thing to do to keep the engine happy is to get the primary balanced. Factory balance on Polaris clutches is sketchy at best.
Price seems pretty high, but there's not much market for that engine around these parts.