Do you know who the bidder is? What can you find out about him/her/them. If it's a big company, maybe you can ask for more? If it's an anonymous offer (via Sedo for example), is it an initial offer? If so, maybe you can manage to get a few more G's, but it's always a risk that the bidder will change his mind when you counter. Does it get any particular traffic/revenue? It's a very generous offer IMO and if you can't risk "losing" those $6k, accept the offer.