Pick a store and keep it up in one tab and then the shop search in another and you can go through and look up the items so you know what has a profit and what doesnt
if you stick to the same store you will eventually learn what you need to buy and what not to buy :)
What said. Also, items that have more than 1 in stock are usually not as rare = not as expensive either.
And the items are ordered by their ID, so the newer items (usually the most expensive ones) are the last ones.
A good, but slower way to restock is to do quests, and if you're asked for an item in a store that's clearly selling at a profit, buy an extra copy of that item and then resell it. I have friends who say they have no patience for restocking who do fairly well with that strategy.