I think there needs to be a clarification on what O.E.M. (Original Equipment Manufacturer) and 3rd Party (Knockoff) when it comes to accessories.
Wikipedia definition of OEM is as follow:
"An original equipment manufacturer, or OEM, manufactures products or components that are purchased by another company and retailed under that purchasing company's brand name.[1] OEM refers to the company that originally manufactured the product. When referring to automotive parts, OEM designates a replacement part made by the manufacturer of the original part.[2]"
So if you are listing a camera battery and it is not branded as Canon (came in Canon branded packaging) then it should not be listed as OEM.
Everything else is either knockoff or 3rd party and should be listed as such.
I hope this is added to the rules of buy and sell.