The seller is lying. How could you not know your own firmware password? What you may be looking at could also be an activation lock put on it by the real owner after the device was stolen. If they did actually "forget" it, then that Mac must have been laying around for quite a while since it couldn't be used. Which means they made their forgetfulness someone else's problem by selling it as is.
No, Apple will not unlock it. An eBay, Craigs List, or any other person to person sale is not proof of ownership. You have a receipt that shows you paid for it, but that doesn't mean it belonged to the seller to start with. If Apple did unlock it, two things are an issue. One is they simply unlock it, and all personal information on it is open for anyone to look through. They won't do that. If they unlock and wipe a stolen item to remove such personal data, then give it to you that way, then Apple is implicit in trafficking stolen goods. They won't touch it.