mob/ShopKeeper
verb/Buy()
set src in oview(1)
switch(input("What do you want?","Potion Shop") in list("Potion 1","Potion 2","Leave"))
if("Potion 1")
//stuff for buying potion
if("Potion 2")
//stuff for buying potion2


thats a simple potion buying. I hope you can do the stuff for buying... well, since your new, i guess not, so...

if("Potion 1")
if(usr.gold >= 10)
usr.gold -= 10
usr.contents += new /obj/potion
else
usr << "You need 10 gold to buy a potion!"

Thats fairly simple, all you need to do is create obj/potion, and have a gold variable for mobs