I gave the skill a bit of a tweak. What you should see now is, in no
particular order:
a) The chance of fumbles is now tied to your skill percentage. It's still
possible, though, but a bit more rare for those with high skills.
b) The exchange you are about to do affects the success percentage. It's
fairly easy to go up to gold, and then it gets progressively harder.

Rgds, your friendly neighbourhood Sage
PS. This is not really an up- or downtune.