First of all I'm extremely proud of the maturity level in this topic. Really, I'm so impressed :)
As previously stated by Genesis, the economy atm is well. I really think that this kind of thing should perhaps only be implemented if the economy starts to decline. Or maybe I'm wrong and it would be more beneficial to implement this before anything wrong in the economy happens. And yet on the same token the very NPC that would balance our economy could potentially destroy it. Which is why the choice of items needs to be well thought out.
Might I also add, that Genesis could add a quota on how many of each item a play can buy from the NPC per day, or per week etc. Inevitably this will cause an issue in players making extra characters in order to buy the items without restriction. My proposal to inhibit too much of this is to have a high level requirement. Let's be honest, leveling on a high rate is not hard work, despite the number of newcomers who beg for leeches. But newcomers will not have the coupons to begin with anyways (unless they donate a lot of money)
The NPC would sell custom items.
Here are a few things I think would be alright, if controlled properly:
Speed pots
Converters
Starcrumbs
Foods
Disguises (like we had at the Halloween events)