In addition to a couple of excellent answers, I'm going to say designer clothing.
I love magazine ads where a model is wearing a plain T-shirt that "only" costs $250, or jeans that cost $500. I lose my mind paying $45 for Levi's on sale; you'd have to be on crack to buy jeans for $500. Not even if I was a billionaire.
I grant you that some stuff that I buy that is on the expensive side is actually worth it. UnderArmour hi-tech fabrics really do work, and they are worth the money to me, not because of the brand name, but because of the function. But I buy generic versions when I can.
CONDOMS :-D I think these should be given free of cost from the tax payers money and delivered free with every news paper even to those who don't subscribe to news papers - every government's effort to at least beat STDs where one can
As of fragrances or designer clothes or cars or any such stuff, I LOVE them and think every one of them are priced for what they are worth. If I think they aren't worth it, I won't buy them :-D. it's the brand that sells and brand comes from quality that goes through a lot of research :)
Since you had questions about the diamond industry (aka De Beers), you should read this 1982 article. Nothing substantial has changed; the De Beers corp has continued to buy out any new diamond finds that are discovered so that they can maintain tight control of the market.
An overpriced product only stays on the market briefly. It's a self-correcting problem. Perfumes are "overpriced" because people pay what sellers are asking. They are by definition, then, priced appropriately.
Apple products, ANY type of clothing at ANY mall, Starbucks anything
Starbucks coffee, the building is literally right by my apartment area. I never go there unless my cousin forcibly grabs me by my arm and drags me over there. I just get sad a little in the inside that I'm wasting my money on a drink that I can go buy at Wal-Mart, which is literally right beside of my apartment! and get coffee ice cream and mix it in a blender, I get more out of it and less money to spend :) Even their desserts are overprices at Starbucks, they don't even taste that great for one thing that is like 3 bucks? they better be organic or super healthy if I'm going to pay for a small snack that is 3 dollars.