I agree $800 for a 15 yr old machine sounds a bit high. I forget what these machines were going for new. It was way more machine than I was interested in at the time. I purchased a lower model Husqvarna for $1200 new about that time frame. I believe list on mine (Platinum 950) was about $2000 at the time.
Can you check with the dealer where it was likely purchased new?? Perhaps they can tell you list price at the time. You could get some pricing on a new machine (same manufacturer) now to get a feel for how much they typically discount??
And lastly, is this a private online sale (craigslist or the like?) or is it from a shop? From a shop I'm sure the price would be higher. And keep in mind as well, just because that's the price they're asking - it doesn't mean you can't offer a lower price.
FWIW - I love my Husqvarna! I've rarely had it serviced and have never had a day's worth of trouble with it.