Are questions on numismatics on-topic?
Coins are, and have been for a very long time, quite literally, money. On the other hand, the value of rare coins stems from their rarity and the fact collectors want them, not from their face value. In that way, their value is no different from the value of rare stamps, which I would not consider asking on Money.SE.
For example, I may consider as a question:
I've received a commemorative coin as change when buying something on a market.
Is the £1000 sale essentially a fraud, or is there some reason why the value of this coin may vary over three orders of magnitude?