What you require is at least as easy to do in Quest as in any other I.F. Language - exactly how you'd go about it depends on the level of sophistication you require.

The most obvious way is to just store the "credits" in a numerical variable, and increment / decrement that as they are consumed or replenished.

e.g. You have a numerica variable called 'playercredit' set to 100. If the player buys a sword you might reduce the 100 by (say) 25.. if he sells a bow you might increase it by (say) 40. It is simple maths to then work out if a player can afford to trade.

Sorry if this is vague and non-specific, but without a concrete example of what you want to do it is difficult to be precise!

Al