1st Dibs has a new policy where they hide the name of the seller. My guess is that this is to prevent buyers from going off the platform as this way the item can likely be had for less than the 1st Dibs stated price (given their markup). However, my preference would always be to know the name of the individual reseller so that I can research them to my satisfaction. So, a big boo to you, 1st Dibs.
This ring (offered by a now mystery dealer in NY) is described as “A platinum ring showcasing a cabochon Burmese blue sapphire – non heated- weighing 18.91 cts, sided by two half moon shaped emeralds. Circa 1965.” Its selling price is USD$150,000. For that money, I definitely want to know the name of the seller with whom I’m dealing.

That’s gorgeous! I agree, 1stdibs markup is ridiculous. They listed a Weiss Christmas tree brooch this year for $350 that you can find everywhere for $75-150.
I believe their markup is in the vicinity of 40%, which is one reason they likely masked the individual dealer names. Too easy to go off site and get a better (more realistic) price.