For online sellers, I do not bother. I hardly buy anything online anyway.

When it is at a market or a shop, I might strike up a conversation and try to see if they are genuinely interested in what they are selling. If they are, I may let them know what they are misrepresenting. However, I will always do this without any other customers present. Nobody wants to be 'schooled' in front of (other) customers.

If they do not appear interested, or I get the idea that the deceit might be (somewhat) intentional, I ignore it. Generally not worth my energy.

Once had a high end Dutch antiques dealer with a very good reputation grossly misrepresent beads (new Indonesian repros). Started a conversation but was quickly 'put in my place'. Not wasting my time. I'm sure they will have issues with customer complaints when the customers find out.

Sometimes, the lack of knowledge of sellers comes in handy as well. Bought several items cheaply when a seller did not know what he had. Silver and saphiret necklace, Egyptian revival Czech necklace, etc.