For Future's market, the description says that there is a 50g lending fee per item.
However, how does that work?
When I have 0 gold and I buy a Faerie Charm, I'll be at -125 gold.
But then what is that 50g lending fee?
That makes me expect that if I buy Faerie Charm when I have 0g, I'd be at -175g after the purchase.
Since I'm not, I'm assuming that either the Rune isn't working correctly, or the description is wrong/unclear.
Tested it with MikuMiku's suggestion (thanks) of leaving the Fountain and then it does subtract the lending fee, except that if you buy 2 items that require you to go into debt (so two Faerie Charms that together put you at -150g), you still only get dinged for the 50g lending fee once, even though the description clearly says 50g PER item!
Edit: I meant this for the Bugs board >.<
Edit 2: Added updated info.