Can you give me an idea of how this looks on a website photo such as Etsy or Ebay?
That is a difficult one to answer. It basically depends on the light that you use and whether your subject is close to or far from the background. If you know the inverse square law it basically means that the light falls off quickly the further you are from the subject the less light will reach it. So your grey background can even turn out to be black.
Probably not the answer you were looking for, but there is no easy answer. Your flash or lack of light is either going to spill onto it or there will be no light and it will get darker or lighter shade of grey if you are close. Sorry I cannot give you a definitive answer. you will have to experiment. If you get it right, 18% grey is what it will be and stay the same color. Best of luck.