Would this lens fit on my Nikon d3100?

Http://www.ebay.com/itm/380373911941

I have my doubts that it is a genuine Nikon lens

either way, yes it will mount, but it will not meter, and will definitely not autofocus

Series E lenses were older Nikon budget lenses of the late '70s vintage. They are optically inferior to the normal Nikon F-mount lenses.

They are manual focus and you will have to also use your camera in manual exposure mode as well.

If you are interested in a 50mm lens, buy the Nikon AF-S 50mm f/1.8, which has a MSRP of $219, so it is not an expensive lens, and not worth buying the one on eBay.

It will fit being an F-mount lens. It will have to be manually focused and the aperture will have to be manually selected. I think your camera's lightmeter will not work with it so you can't use it in P, S or A modes. Only M mode will work but you will be completely on your own.

That is definitely a genuine nikon lens. It will fit but you will have to do everything yourself so M mode only. If you can live with that you have a fine and rather sharp lens for not much money.