So let me get this straight, Thoughthammer has a section that they sell games that are not in good condtion?
This is an established practice. Funagain Games is another company that does this. And a 2-second "Boardgame Ding Dent" Google search reveals more businesses that sell discounted merch.
Generally it's because the package was creased or crunched or damaged during shipping. Or because the shrink wrap was slashed. If it's Thoughthammer or Funagain or another reputable business in the community, then it it's in their best interests to accurately assess why a game is being discounted. And this sure beats receiving a "new" game with broken components inside.
As to whether you should buy a Ding and Dent - it's your preference. Some people don't like any imperfections in their game copies and treat them like first edition books or antiques. Others recognize that as playable copies they may suffer some shelf wear or accidental dings and dents. Your mileage may vary. But the dealbreaker shouldn't be because you've never heard of the Ding and Dent discount being a thing.