I've been buying vintage jewelry for a very long time. Most people who sell this stuff are just ignorant - as opposed to shady or malevolent. I've seen a variety of beads called "jet" and
"jet glass," when it is simply black glass. When I've told them (politely) that it wasn't jet, their reactions have run the spectrum from outrage to complete embarrassment. Many of those flea market and "antique store" sellers are now on eBay or selling on other sites, and almost none of those people specialize in vintage and/or antique beads. As far as jet is concerned, unless they have some knowledge of Victorian era jewelry, they rarely know the difference between jet and black glass.

I've found that, if you're buying online, it's a good idea to check whether the seller seems to have some bead expertise (for instance, sells a lot of beaded necklaces). Unless, of course, the photos are outstanding and you really know your beads.