"I've got a bridge to sell you" is an idiom used to indicate that one believes someone is especially gullible. Gullible mean easily persuaded to believe something. If you'd like some examples you're welcome to send me a message, I am available all day.

Teacher V is correct. Someone actually committed fraud in the past, selling people shares of the Brooklyn Bridge (which he didn't own of course). So now it's kind of a saying.