Isn't there a very simple fix for abuse prevention via alt accounts? You can only trade leftover artifacts for event buildings you already got to stage 10 in your town.
Meaning, everyone can ask for artifacts for a building they want to evolve, but can only offer those for buildings they already finished in return. That also means, you need to have all 3 old phoenix buildings at 10 before you can give away the artifacts you get in the spire or via crafting menue.
You could even open the option to transform artifacts via crafting with this restriction in my opinion.