I was actually offered £400 for my domain name once ( scrim.co.uk ) which surprised me. If it's worth £400 (which I actually doubt), then I would imagine that the windowcleaning.co domain is worth more that the £500 requested. But, like anything else, something is only really worth what someone is prepared to pay. BTW. I declined the £400. Although I've been lazy about developing a website, my email is heavily linked to the domain and I didn't want customers struggling to make contact by email.
Just checked online and my domain is valued at $2,300. Hard to believe it though. The site is an auction site so are probably trying to encourage people to put business their way.