Without knowing actual print run totals, a rarity guide would be more of a rarity guess. That said, a trip to PCE Daisakusen gives a decent idea of a title's scarcity and will only get better as more users register.
I suspect that what is being asked for is more of a price guide, in which case ebay is a good resource. As long as you're looking for common titles and only look at completed auctions, the average sale price is fairly accurate, though the real value is a bit less after deducting ebay's exorbitant fees.