I'm always amazed that someone would sell a perfectly usable OEM exhaust (even if it does have a couple of cosmetic scratches) for a ridiculously low price of $25... heck, even $100 is dirt cheap for an exhaust that you'd have to pay $600 or more for a brand new one from Honda.
I have to wonder how often someone buys a cheap, low quality aftermarket exhaust for their bike, and then practically gives away the stock unit for what amounts to the price of a case of beer, only to have the cheap aftermarket unit blow out whatever little packing was put in it at the factory in China (plenty of stories of crappy Chinese aftermarket clone mufflers sh!ting the bed in short order over on the CBR250R forums). Talk about a 'kick yourself in the ass for doing that' situation.
Personally, I'll always have space in my garage to store the OEM exhaust's from my bikes... if for no other reason than the fact that the day will eventually come when you can no longer buy a brand new OEM exhaust from Honda for that particular bike. At that point it becomes a "sellers market", and that's when someone who is restoring one of those bikes and needs that exhaust, will likely pay big bucks for it on e-bay.
That said, I don't see any one paying $400 for a CBR300R OEM muffler right now. It's not 'rare', and I doubt that there is much current demand for one... perhaps in 20 years it could be worth $400.