Help with pricing : Radical SR3 Wheels

I'm wondering if the experts on this forum could give you some insight. I've been looking for another wheel set (or 2) for my '07 SR-3. I've come across someone with 4 sets of actual Radical wheels (15" fronts, 16" rears) and he's asked me to make an offer.
I don't have any experience buying/selling Radical wheels...what is a fair price?

Thanks much,

Rick