Saving Event Pokémon?

Hiya! So I've been catching lots of event pokémon and while a good portion are 3 stars, I only have one "extra" with a special background and only one shiny I plan to keep. Are they worth keeping for trades (Charmander, Charizard, Pikachu w/ captain hat etc)- like are people generally interested in them even if they don't have a special background? I've kept a ton of special background Froakies from the previous even too for future trading but all these are taking up valuable real estate.

TL:DR; what're the protips on keeping event pokémon for trades or not?