That listing is for a Tamagotchi Mini. The mini is
not an original Tamagotchi. The Tamagotchi mini is actually a truncated version of the original an is around 20% smaller. That listing is a genuine Tamagotchi mini because it has the Bandai logo and also it is being shipped from the UK, which currently is the only place that has them in-store.
This website lists most of the official Tamagotchi shells and the versions you are looking for are found under the section vintage, labeled P1 and P2. The P1 is the original Tamagotchi while the P2 is the second release of it featuring different characters. Both the P1 and P2 come in similar shells but the key difference is that P1 Tamagotchis have a
checkered background while P2 Tamagotchis have a
swirled background.
It is quite easy to find a genuine Tamagotchi by keeping in mind these principals:
1. All Tamagotchi versions have only
three buttons. If you see a device with four buttons or less than three, it is not a Tamagotchi.
2. All Tamagotchis have a
single shape. If the shape of the device matches none of the pictures on Project Tama Shell, the Tamagotchi Wiki, or official promotional images, it is not a Tamagotchi.
3. All Tamagotchi packaging contains the
Bandai logo. If there is no Bandai logo on the packaging, it is not a Tamagotchi.
You will avoid most fake Tamagotchis by just following these principals. The only thing that fake original Tamagotchis may have is perhaps a different background. There are only two background styles for original Tamagotchis (which you can find in the Tamagotchi P1 and Tamagotchi P2 sections of Project Tama Shell) so if you find a device that has neither, it is fake. One other thing to note that the gallery for P1s and P2s is not complete on Project Tama Shell so if you find a vintage Tamagotchi that matches all characteristics but the colour is not in the gallery, it is not a fake.
Having said all that, Tamagotchi fakes are rather easy to spot so there is little danger in accidentally buying a fake Tamagotchi. Furthermore, most knockoffs don't use the name "Tamagotchi" at the top of the packaging but may instead use various generic or bizarre names. If you are very unsure if the Tamagotchi you are buying is genuine, simply post in the "What's On Your Mind" forum and someone will certainly help you out.