Whooped out rear suspension and shocks. Clutch/Engine alignment and drivetrain bearings. Good way to tell is using the reverse on the sled, then how it acts when going back into forward. Does it smoke the drive belt? Does it slip? Electric Start issues, starter motor engagement/release..
Whenever buying a used sled, bank on totally disassembling the whole thing, including the engine. Change every bearing in the driveline, rebuild both clutches. Check the crank and balance shaft, piston skirts and cylinders. that is the only way to ride with confidence. It is time consuming and expensive but so is a new sled.
i know people that have two of the big 800's and it completely hauls ass for a big sled but everything has been a problem on them. the rear hand grips on the one caught on fire and torched the one sled, since then the one gear for reverse exploded and ending up taking the chain case with it , the electric start is intermittent with fresh batteries in the sled and controler and it originally had a wiring fire under the hood too. i'd be weary about getting one of these after what i've seen
ps. they're riden by older people that are very particular about their things , they arent abused and are maintained well.