From my understanding, once a price and location is agreed, the trade must be completed. There's really nothing about time to wait or anything like that so mods look for price and location agreement. Not following through can be reported
It's a little annoying but it doesn't seem to affect auctions where it hinders the pokemon reaching a reasonable and desired price imo.
You're right though, it is a way of bypassing the rules and commenting on an auction so it would be great if that was restricted and enforced when it comes to the emojis