What does steam punk mean I've seen it used a few times but I can't understand its meaning
It's a science-fiction movement that imagines how steam technologies –and Victorian styles– might have evolved until today or in the future.
Now, I agree with others, sellers use the term to sell hipster stuff. I see a lot of "steampunk lamps" that are basically rusted plumbing pipes with a squirrel cage filament bulb (vintage replica bulb), as if it was powered by steam! In your case, in the seller's mind the mercury bulb looks like an advanced technology in a very classic clear bulb design, so it would be steampunk.