That's about half what it cost new. This one looks like it needs to be cleaned. I would inspect the firebox and ask the owner to crank it up for you before buying.
As others have said make sure it works and look for structural problems for example rust on the fat drainage channel. Is the fact it needs a good clean masking anything? Are any optional extras (looks like it has a front shelf?) and consumables included.... pellets?
The asking price has already been reduced considerably so there has obviously been no takers, use the fact it needs a very good clean inside to haggle it down starting at say $300 to $325? I still think it is a tad overpriced.
I sold a 15 month old Pro575 and got back around two thirds of the initial outlay, at 3 years old I would expect no more than 50% of initial cost but would expect it to be presented in better condition and sound structurally.