That explains it all - he's not a crook, just a bit dim
It's NOT an HDMI plug, it's a special mini-SCART plug, common these days to save room on the TV as SCART is pretty well obsolete. It's not a converter of any kind, just a straight lead to provide a SCART socket on the end of a short wire.
There are a number of different types, and they aren't usually exchangeable between different makes.
He's totally (and accidentally) misrepresented the sale, and should give you a full refund without question.