Sometimes, I don't have enough gold for a pretty pet that's on sale and I ask the owner to reserve the pet for me if they can and when they do, I forget later on the pet's ID and stuff and I can't find it again. It'd be nice if there was like a "Reserved Pets for You" list in your account or something. :) Just a suggestion.

If you want to keep track of that stuff, I'd say either keep records from the PMs or use the space on your about me section as a "Note to self" kinda thing. Lots of people do that to just keep track of reserves and mental notes.
When you ask the person to save the pet for you, just make sure that you link the pet. As long as you keep the message in the Sent box, you can always bring it back up.

Yeah, I guess XD I'm just a lazy buttface and I usually do that in gen chat :p

I just make a notepad or document and make notes and save links in them. But the ideas above are good, too.

On the selling end, I kinda think the coming pet profiles will be a nice way to help keep track of things eventually. So if you forget about a note you made you can see it on a pet before you maybe release it or try to sell it.