Shops

Shops will be left mostly to the DM's discretion. However I use a few general guidelines: As for potions I usually do:
 * 1) When entering a shop, I'll roll 1-20 for each party member.
 * 2) I then use the same process for creating gear that I specified in Loot Drops.
 * 3) I then set a general price of nr_rolled gold + 5*nr_stats. So if the item had +1 agi and +2 stamina and the roll landed on 18. The price would be 18+5*3 = 33 gold.
 * 5 gold for a small potion, restoring +4 of a resource.
 * 10 gold for a big potion, restoring +8 of a resource.
 * 30 gold for a panacea potion, fully restoring all resources.

Haggling
You can always try to haggle on an item with the following limitations:
 * Each player can only haggle once, and each item can only be haggled on once.
 * The DM rolls the player's charisma number+1 and the nr it landed on minus one is the amount haggled.
 * Ex: The thief has 5 charisma and wants to haggle on a dagger. The dagger costs 10 gold. The DM rolls 6 (cha+1). It landed on three. Which means the merchant is willing to sell the thief the dagger for a new price of 8. The dagger can no longer be haggled on, and the thief cannot haggle in that store any more for a while.

Selling
When selling I usually use the pricing calculations above and roll for that price and add a charisma+1 roll-1 to it. Which means some times the merchant might want to buy it for full price, some times not at all. A general rule is that a merchant will not buy back an item they just sold you for a higher price.