A few immediate thoughts:
As a student, I don't care about a two-sided platform (which is what it looks like you're trying to build). I want to sign up for vetted piano lessons through your site, and purchase a USB midi adapter that you recommend. As a short-term MVP, perhaps point people to a known-working adapter on a site with an affiliate program. Long-term you might consider finding a company willing to sell one white-label and put your logo and website on it, but that's a lot less important than everything else you're building so put it off for a while.
Also, for the first handful of lessons and for subsequent practice, it'd be awesome if you had automated, pre-recorded lessons available, to get the basics down, or to train things like reading sheet music at speed.