Unfortunately, @pm030659, you cannot list an item as a particular brand unless you are certain that it is genuine. Failure to do so will put you at risk of claims of SNAD, and of your items being pulled by eBay, as well as receiving chastisement from eBay.

For those about which you are not certain, you could list it as egg-shaped computer toy, or Egg watch (computer toy). If you're not sure, you cannot include the name Tamagotchi in the heading or in the description.

List the ones that you know are genuine as Tamagotchi.

This is the only way to avoid pitfalls.