A diamond dealer in beverly hills finds that when he advertises his diamonds on television he sells more diamonds although he has secretly increased the prices. this rise in sales, he says, clearly indicates that demand is upward and not downward sloping.

I think the more advertising he has makes the diamonds market value higher because it gains popularity at the same time it increases the demand in the market. This is different from the law of supply and demand, which should make his product have a lower price. However, the focus was only on his product without any competition going on at the same time. That also gives him the power to control the price.