If we were to produce a Buyers Guide, would we sell it/only available to club members? It would need a prospective buyer to find us, while it seems vendors/purchasers usually don't involve us.

Insurers must surely be the ones who have a professional interest in correct valuation? I don't know how their knowledge / opinions could be accessed, but it might have more credibility than that of a man-with-a-spanner-in-a-shed-in-Nuneaton (as we were once infamously described)!