Lucky you! I imagine you are aware of the retail price of these. Dont make an offer. Ask them how much they want for it....then act disinterested. Make a ridiculous offer and when they say no, thank them and head for the door. Just before you leave, turn around and offer an extra $100.
Youre unlikely to lose out:
a) its unlikely someone else is going to walk in and buy it, and,
b) even if you end up having to offer a lot more than your original offer, they arent going to say no because you tried to get the bargain of a lifetime.
In the final analysis though....what is it worth to you?