Starting a marketplace

Hi everyone! Long time lurker, finally going to try and build something.

I have an idea for creating a resell marketplace for band merch/vinyl. Think something like Grailed, but with a focus on artists and bands instead of clothing brands. My research leads me to believe there is an untapped market for this. Other than Discogs for vinyl, there doesn't seem to be another marketplace for vinyl/media specific resell. Apart from this, if you're looking for band merch, your best bet might be ebay/depop, but these don't focus on that specific market. I know there definitely is a market for this, as I've joined facebook groups for band merch resell with tens of thousands of members.

Now my issue is that I'm not sure how to go about market validation... should I create a landing page and post to subreddits/facebook groups to gather emails of those interested? Should I build out the mvp of the product, then share and hope people start using?

If any of you have any experience building out a community or marketplace, let me know! This feels like a big endeavor but I'm excited about trying to take it on.