...So, it would just be a sequence where you'd just have a bunch of branches asking for an item for a price?

<> Message: "I want to buy a Potion for 300 GP"
<> Choice: Sell/Don't Sell
...<> Get gold tally
...<> Take away a potion
...<> Blah
:End

<> Message: "I want to buy an Invincible Armor for 100 GP"
<> Choice: Sell/Don't Sell
<> Etc.

Then at the end, just tally what you have and don't have..

Again, I've never actually played the game, so I don't have a clue of what you're looking for.