When you mail an item to someone in the inventory, you can just type in their username; when selling a pet, you have to type in their ID. Just why?

This feature would also be convenient if the recipient had a very long ID, as most people who are newer in the game have. When I was new to the game myself, I didn't know how to sell to a certain user because I wasn't sure where I could find the user ID.

Of course I see a downside to this, as people can change their names every 180 days. Maybe the pets could be sold to username or ID. I also see that it would be extra coding for Krinadon, and I know that coding is sometimes undesirable.
But even when people can change their username, there's only going to be one user named Arctic - even if I changed it to something like "alsjdflkdj," there's still only going to be one "alsjdflkdj."

My concern is that reserving a pet for breeding is very similar to reserving a pet for sale so changing one would probably change the other, unless you want redundant code.

For breeding, people changing their usernames could be very problematic. For example if Player A promises Player B breeding with a pet for B's project, but the project takes a year and B decides to change their username in that time. I just feel like this feature will end up causing more inconveniences in the long run.

Mostly, I don't support since I feel like the current system works well enough and Krin probably has more urgent coding to do (like getting the fests semi on time)

no support, way too much work to change something that works fine already