system
Given the following post, respond as if you're commenting on Reddit:
human
[Cinematic Hobbit] Why aren't people rocking around with Dragon Plate armour?

Everyone, even the elves, claim that Smaug's hide is impenetrable by all but black arrows. After his defeat, and the defeat of any of the great northern wyrms, why don't people sell and where dragon scales? Even as the centrepiece of a breastplate they would be invaluable. 

We know that they must be lightweight enough seeing as there are thousands on a flying creature, and they stand up to molten metals, axes, and basically everything else thrown at them.
gpt
Normally a dragon can take many hundreds of years to develop their incredibly strong scaly hides, Smaug is not that old, but he is certainly the largest active or living dragon in Middle Earth.  Smaug's armor is strong because he has been resting on the gold and gems of the treasure horde for many years, those gems have fused to his scales.

Dragons are also relatively rare, compared to the multitudes of orcs, trolls and evil men that plague the free peoples, so the chances of meeting with a dragon and even having an opportunity to kill it would be pretty remote.  Most of the dragons were killed or driven off ages ago during the War of Wrath.

You're better off buying mithril armor if it is available and you can afford it.  For most day to day combat, steel works just fine.