If the object being weighed is a fake, no amount of weight on the other side will lift it. I would then let the PCs hear a story at a local bar of some higher level adventurers who are looking for said golden scales for a quest they are undertaking. Perhaps they offer to buy the scales, or perhaps they enlist the parties aid in the quest they are on. Or perhaps they try to steal, kill the PCs for it. The main point is, if you make it magical, why not also make it a story hook!
This is a good rule of thumb which should always be at the front of your mind: If you're going to give the PCs something, make sure the story comes first. This above all. Even with random treasure. Never just give someone a +1 dagger. Make it a dagger fashioned by elves long vanished into the mists of time, clearly magical, which never rusts and never needs to be sharpened.
I like magic items which are useless in combat. All the stuff in the DMG is useful in a game-mechanics context: It lets you hit, see, avoid, etc., it has an impact on combat and skill-use effect determination. Surely there's more than that. I figure, if the world is magic-rich enough where people make Cloaks of the Manta Ray, why not make it magic-rich like our world is technology-rich? We use technology to make life easier. Why wouldn't a magical fantasy world do the same, just with magic? Just as not all tech in our world helps you kill bad guys, not all magic should in our fantasy worlds. Things like slippers that keep your feet warm no matter the ambient temperature. Brooms that do their own sweeping up. Little pellets that burst when you throw them, not turning into
dust of sneezing and choking or
cloudkill, but that instantly remove dirt from a 4' radius. Bookmarks that remember your page when you fall asleep while reading. An axe designed for woodcutters which won't cut living flesh, only wood.
In your case I'd make your scales one of those useful magical items: When you place an item on one side, a magical weight appears on the other side upon which is written the weight of the first object. Think digital postal scale but tarted up with swords-and-sorcery. That could also be an adventure hook, especially for low-level PCs. They found this thing which will be incredibly useful to a local merchant. He tries to buy it. They get suspicious and jack up the price. So he arranges to have it stolen. Hilarity ensues.