Don't worry about it being engraved ... all US stamps from this era are engraved. This stamp is authentic - it is the gum that is suspect.
Here's a link to 1847usa.com about stamp-printing.
http://www.1847usa.com/ArtOfLineEngraving.htmIf you are new to 19th century US stamps, you are in a good place here with SCF. There are some really knowledgeable people here, and if they don't know the answer, they will likely know where to get that answer.
Back to your stamp.
I see a spot on the lower/left that looks like a gum disturbance.

This is likely what the seller is calling a 'hinge' mark. That could very well be, but I am ever the skeptic. If you already have this stamp in your possession, you can send it to William Weiss for certification. $20 gets you a nice certificate. If you are considering purchasing this stamp, I'd advise against it ... but it's only one opinion. Get other opinions.
Brian