I restock at Pharmacy, Kayla's, and Magic. I would not recommend the latter two shops for beginners (items go really fast, especially in Magic
), but if you are fast enough, you can get really good items from those shops and make millions quickly
As for which items you know to buy, I usually look at what items restock, see which ones go really fast, and check the 'wiz to find which items are profitable. I also ask people from my guild about good items, as well as check people's restocking screenies to find out which items are unbuyable or really good to buy