Here is an idea to use loyalty and reputation.
Once you start hiding these stats and gold, I think it would only be fitting to allow us to know how much gold each party is willing to spend in your shop.
Let's say Northmen with 1000 gold visits Penchant's shop. Penchant's shop has -40 loyalty with them so Northmen tells Penchant the party is willing to spend 100 gold out of their 1000. Of course they will not tell him their max gold. They roughly tell him what they desire to buy then moves to De-Legro's shop where they tell De-Legro they are willing to spend 800 gold because De-Legro has over 100 loyalty. Also, they are willing to spend another 100 gold for De-Legro's good reputation.
Also, I think there should be a cap for loyalty. Maybe 200. Each time you benefit more than the default base*1.2, loyalty falls. Make the fall bigger if you are close to 200 but less as you reach 0.
