Yeah, the fees are insane. That is part of reason that I wanted to try a different blockchain, but all the non-eth platforms come off as unprofessional to me.
Yesterday I tried to join MakersPlace and like you say, it looks to be invite only. I noticed @reinhard-schmid is on there! Cool. One of my buddies, Steve Varner, has some serious industry cred but is not a household name. We shall see if I can get him hooked up.
I get it though, and don't blame them for being picky. The problem is, if you don't vet people, you end up with a big mess of crap to wade through. I am noticing this on Opensea already, which as you say is still open to the general public.
update: oops, it wasn't Makersplace that I tried to sign up for, now that I am looking. It one of the others that just took my email and said they'd be in touch. Apparently I already have a Makersplace account... Don't even remember making it.
MakersPlace is a great platform. I didn't make my best sales there but others migh have.