This site is an amazing tool for stalking the auction house efficiently when you can't be directly in front of an AH. Combined with the Remote Auction House, this allows people to keep a semi-persistent presence on the market without needing to be in game all day. It's also invaluable for researching new markets and strategizing against your serious competitors. In fact, the only feature people have really voiced a lot of concern about is the seller activity page. The heat map can tell people when it's worth logging in to undercut, which can lead to people feeling like they're being unfairly targeted. The current items section tells interested players what other markets they could target someone in.
The standard response to concerns is that this is all just data, and what people do with it is not the fault of site that makes the data accessible. There is no privacy in an open market, and just like you can hardly be upset if someone puts a photo of a sale sign in your storefront window on the internet, you can't get upset if a site like the UJ makes everything you have for sale available through a public interface.
Personally, I like being able to stalk my competition more than I'm afraid of what they might do by stalking me. After all, there's only one of me and at least 50 of them. Additionally, I like to practice what I preach about market agility, and I like to think that every time a competitor closes a door on me, I can find a new door.
Maximize your profits with more advice from Gold Capped, plus the author's Call to Auction podcast. Do you have questions about selling, reselling and building your financial empire on the auction house? Basil is taking your questions at firstname.lastname@example.org.