400 Euros a month is a hell of a price to find out if they're worth it. Can you see the prices without buying a membership? You'll have to target something like $1,500-$2,000 a month in sales from that single supplier, in order to break even on that one supplier. Of course, that's not a huge amount of sales; just saying it's quite a bar to jump over when all that volume is doing is subsidising your membership fee.
Information available online about BD seems mixed. Lots of people happy with the products, not a lot of people happy with the prices or service. I'm not sure I could personally tolerate subpar service when I am paying over $500 a month for the privilege of being a customer, but of course I don't work that niche and don't know what kind of volume you anticipate doing. ie. many of my own misgivings about the site stem from the vantage point of testing or just starting out in a niche, where a $500+ monthly membership fee would be prohibitive. Established sellers and people who really know the niche may see it very differently.