IANAL but with my 10 mins of googling I've found the following:
* Items/characters cannot be trademarked unless they are the source of goods or services (so therefore this service would have the right whereas JK. Rowling et al wouldn't) [1]
* Items/characters are copyrighted when featured in a copyrighted piece of fiction (which Horcruxes are). [1]
* JK Rowling et al are crazy litigious [2][3]
* JK seems to endorse/praise fan collections of stuff, but the team come down hard on people trying to sell that commercially. [2] That does, however, seem to be for publishing books eg. fan fact books.
* Items/characters cannot be trademarked unless they are the source of goods or services (so therefore this service would have the right whereas JK. Rowling et al wouldn't) [1]
* Items/characters are copyrighted when featured in a copyrighted piece of fiction (which Horcruxes are). [1]
* JK Rowling et al are crazy litigious [2][3]
* JK seems to endorse/praise fan collections of stuff, but the team come down hard on people trying to sell that commercially. [2] That does, however, seem to be for publishing books eg. fan fact books.
[1] https://corporate.findlaw.com/intellectual-property/protecti...
[2] https://www.legallanguage.com/legal-articles/harry-potter-co...
[3] https://en.wikipedia.org/wiki/Legal_disputes_over_the_Harry_...