Sounds like ones credit score!! :) On one hand you have the ability to create a fluid, highly sensitive system that could potentially rate the buyers and sellers through a system of weights and measures. And on the other hand you can look at other sites that have gone the complete opposite route using extremely simple methods to rate their buyers and sellers. I see you have chosen factors that can be weighed universally, which is necessary, but since there are only so many parameters to choose from that can actually carry a universal weight it leaves you wondering exactly what type of customer do you truly have after the computation is complete. I think the parameters you have chosen are good - total amount spent, total purchases, vendors purchased from, purchase distribution across vendors, weight of vendors purchased from, account age.A good way to test this concept is to compare two completely different buyers (one well respected buyer and another not so respected buyer) using equal vendor weights to see how each would be reflected. But yes, the question now is how much weight should each individual factor equal overall. IMHO, I would weigh each in a percentile such as this:-Buyer35%- total amount spent 30%- total purchases15%- number of vendors purchased from10%- purchase distribution across vendors5%- weight of vendors purchased from5%- account ageHaven't thought much about the sellers but maybe we could do something similar with that too. Whatever we decide I just want to mention that I have secured the domain DPRsList.com in anticipation of this turning into the Angie's List for drug dealers. I see a future in this :)