IP is a surprisingly complex area of law, not to mention the potential jurisdictional questions that can further complicate things.
For example, we all know Amazon and Apple are registered trademarks but if you registered Amazon.eth or apple.eth without any additional facts it would be nearly impossible for them to successfully claim you are infringing their trademarks. This is do to the nature of the words Amazon and Apple which are descriptive and generic. On the other hand if you created some website on Amazon.eth or apple.eth that either confused the public into thinking your websites were official websites of Amazon or Apple, or if your websites were otherwise harming the goodwill associated with those marks, then Amazon and Apple would likely have solid claims against you for trademark infringement.
Unfortunately due to the centralized nature of OpenSea remedies are limited. You could contact their customer support, review their terms to see what procedures are in place if any, hire a lawyer to respond to a takedown request, but for the most part OS is free make decisions independently and run their platform as they see fit.
As a funny side note I registered geocities.eth and because Yahoo’s TM for GeoCities was cancelled in 2020 I decided to file a trademark application for GeoCities with the USPTO using my ENS name in part, and in February of this year I became the owner of the GeoCities trademark. After obtaining the TM I submitted the TM to Twitter requesting the @geocities handle (which has been suspended for years). If @geocities was actively being used and infringing my mark I could file a takedown request which I’m sure Twitter would comply with, unfortunately so far Twitter seems hesitant to process my request and give me the suspended @geocities handle. It would be ridiculous if they don’t give a suspended handle to a lawful TM owner, but that is their right and just highlights one of the problems with centralized platforms that makes decisions behind closed doors.
Out of curiosity can you share the ENS name that was take down?