I was going to sell my starter, Mercy the Zolnixi, but I found out that you can't. I was just curious about the reasoning behind this. Is this so other people can't make multiple accounts with Starter Pets with rare traits and sell them to their main account?

There may be additional reasons, but I think it is mostly to deter multi-accounting for starter pets.

Krin has talked about adding in the future a kit that if bought can make it where you can redesign your starter pet. However, it'd be the same species. This is particularly nice for people who joined early on before many of the current traits are available. Also, you can join and not really know yet what you really like. I stil hope though eventually for a way to change species, too, as tastes change and also after revamps you can find yourself not as attached as you previously were. Maybe an even more expensive kit. I'd love to make mine into a kelpari.

Okay thewn nvm :p

To clarify to Grapejuice, no, your user level corresponds with the highest level that you've raised a pet to (any pet- if it gets a level up that takes it to a level higher than your current one, your level advances to that level).
It just happens that for many people their starter will spend a lot of time on their team while they get used to Sylestia, so I see where the misunderstanding comes from.

HybridUnknown, yes, pretty much a lot of it is to prevent the multi-accounting. You can release your starter, though, but only after you get to Umbra Forest, if I am remembering right.