Well, if it's a recurring feature of the shop (to have limited stock), then it's better to use SRDude's plugin indeed.
Also, I seem to remember a game in which you could see how much money the shopkeeper had, so the player knew exactly how much he could sell on that specific shop. Maybe that's something you could look into.
No worries. The offer is there if you need any help. I figure making a whole game on your own can be quite a challenge, and if you ever feel like sharing the "burden", I'm willing to help. If not, I'll be enjoying the game when it's completed. Hahah
Since making maps is the part I enjoy the most, I'm much further on my own game than, say, eventing and even writing.
Yesterday I spent the whole day figuring out a formula that worked for my damage calculations... and man, was it a pain! xD
Anyways, cheers!