Wait, is that right? You're supposed to refill everytime you buy a card or kill a monster? Crap, that certainly changes things a bit.
It explicitly states that you do for monsters, it doesn't at all for purchases. It has been said multiple times in the forums that you do.
I plan to ask Kevin Bender who demoed this at Gencon if the latter is correct.
Talked with Kevin. You refill the row immediately following each killed monster or purchase.