I agree, as long as it works you can usually fix and/or live with any aesthetic faults. But that reminds me - I did see someone in the Tamagotchi Collectors group post a solution to sun discoloration - if the tamas do arrive discolored you could try it, although it will also lighten the actual design so it might be more trouble than its worth, especially since the Mothra design covers basically the entire shell.
https://www.facebook.com/groups/TamagotchiCollectors/?post_id=1942578189098017
Same here, in my previous collection I had NIB Angels, Morinos, and the Osu/Mesu with the white + hearts shells and they were all perfectly fine! I really think it'll work out
Also, I'm looking at the photo of the listing again and it really seems like it's just the plastic that's darkened. The Morino looks great by the way, I don't think that one will have any trouble.