I'm part of a lot of Facebook selling (and fitnesswear-obsessing) groups and I admin a few of them, too (leggings, Athleta, loose leaf tea, Tieks shoes, Lush). I'd be happy to start a group and/or help with admin duties (trying to make sure people who want to join are real & not spammers -- we get spam applicants daily).
There's already a videofitness group on FB, too. I believe I'm still in it but I haven't been active. Too much time frittered on the other groups above.
It might be easier if everyone joined the videofitness group, and then that could be a good base for the selling group spinoff.
I actually got a sales tax # recently, but I don't sell 50+ items on Amazon in a year.