If everything works I think 300 is fair... if its dirty and possible parts need to be replaced I'd try and bargain down some. Parts are not cheap.. maybe go to the website and print up a list of parts and prices. so you know if the fan does not work it cost so so $ if this looks bad then its this much $ etc..
read up on it on the website so you know what parts are to be there etc.. always helps and then nothing gets forgotten and left behind.