To correct a seller as the the age/vintage of an item they have listed?
The item in question is a brooch being described as art Deco/antique,circa 1930s...yet the manufacturer did not start producing until 1946. The design in question is a current design.
No its not rude if done politely. I have contacted sellers about inaccuracies in their listings on a few occasions. It can save the seller trouble as well as an incorrect listing can give the seller trouble with the buyer and with ebay.