Bullet points on BM:
You sign up for free
Register for $50 - one off fee for lifetime membership this allows you to buy, but not sell yet
Start buying.
If someone is willing to invite you, some or all of the registration fee is waived, but that inviting member is "responsible" for you if you start trying to scam people, or are just not too good of a person to deal with (not to say you will be, but that's the risk the inviting member would have to take).
Also if you are a glassblowing artist, you can jump though a few hoops and get registered as an artist for free.
Once you are able to buy things, you will be a "starter subscriber" most of your posts will be delayed pending mod approval.
After you've bought a few things (I think you need either 3 or 6 pieces of positive feedback on transactions [not "likes" on posts]) you become a "bronze member" which is basically the same as starter, but your posts don't need mod approval.
You can then pay $10 to upgrade to a "silver member" which allows you to sell up to 4 items at once.
I'm not sure what you can do as a gold member, but I don't think they are taking any more gold members, and they seem to mostly be shops IIRC.
And yes, it's totally worth the $50
(if you plan on making multiple glass purchases that is, if its only gonna be a 1 off purchase, it may or may not be worth it.)