A roleplaying session is not like a video-game one: you don't have to have everything planned. Many details don't have to be decided before the PC stumble on them. After all they may never came to this shop.
When players go shopping, you don't have to show them the list of every item available, like you would have in most of the video rpgs. Just play the merchant and make them some offers, based of what the adventurers look like and the items present in your guide. For example if one looks like a swordsman:
Look at that sword, adventurer, it cuts through steel as a knife through butter, and it's only two hundred gold! Oh, you don't have that much? May I recommend you this buckler instead? It's even cheaper!
If your scenario provides informations about his personality, use them! If not it's probably not a very important character so play it as you want.
If they are looking for something specific, let them ask for it, and maybe the merchant has the item, maybe not. Look at your scenario:
is it an item specifically mentioned here? Then you have your answer.
is it a very mundane item? (like a rope, a normal sword or some arrows) If yes then you can assume the merchant has it in stock.
is it something magic/very rare? The merchant doesn't have it. Maybe he can know how to get it however.
is it something quite specific, but not so rare? It's up to you to decide. A weapon seller will probably have at least some weapons of good quality, and if you want you can even make them roll.
Ok, you are looking for a metal cage but this village has no smith since years. Roll this d100, if you are lucky maybe there is still one left somewhere.
You don't have to roleplay every single merchant in the world. If the team goes to a big city and spend a day looking for a rare merchandise in all the shops just tell them the result of their hunt (unless your game is all about shopping).