Valuing a website is difficult, mainly because it's usefulness to each individual varies widely. Would you be happy with $500 for it? If so, take the offer. If you feel that the amount of work put in and the potential it has, minus the work required moving forward is worth more, then wait it out.
If that person is willing to offer you $500 directly, there's probably a good chance they will bid on the same site up to $500 or more if you say no.