How can I tell if an Apple Magic Mouse 2 is genuine?

Mercari is a casual buying/selling organisation. You buy/sell at your peril. Does the business offer you any guarantee about purchases? If not, it's down to chance.

If your purchase was in a sealed box and the serial number was clear, you may have been lucky. Personally, I would not buy without a guarantee.

Well that looks good to me. You can double check by going to Apple support. Because I cannot tell where you are, go to

https://support.apple.com/en-gb