Artstyle is great and I would definitely continue this approach. What needs to be refined in my opinion is the gameplay. For now it is promising, but some form of checking if the item is valuable would be good plus a better negotiation system also would be useful. For now, you can work around it so that you keep on suggesting "1" until the other side comes down to the lowest price.
Maybe a simple system that if the price is too low 2-3 times, there is no transaction.