How do you buy weapons on Roblox?

You must buy them in the catalog. They are known as Gear. You can take gear into places with you as long as they meet the requirements of the place. i.e. Brigand's Sword. It's genre is Fantasy. It's attribute is Melee Weapon. If you want to take it into a place, the place must allow Melee Weapons. However, places may only let in gear with a specific genre AND attribute. So, let's say a place allows Melee Weapons that are under the Sci-fi genre. The Brigand's Sword would not be allowed in with you. Hope this helped!