Did you know it is illegal to sell covers of songs? Cover bands are not allowed to sell CDs unless they obtain license to sell every single individual song on the CD. They are allowed to perform, because you’re paying for their performance, not necessarily for the song.
Is he using Blizzard’s trademark to sell his stuff? The answer is obviously yes, because he’s selling WoW stuff.
Fun fact - did you know it is illegal for artists to sell prints of like Spider-Man an Batman, unless they obtain a license from the rights holders to do so? The main reason license holders don’t enforce the law is because they know not enforcing it is in their benefit. However… how would selling mod parts be a benefit to Blizz? If they could, they would get in on the business, as they did with gold trading.