Hi Christine,
Interesting idea but here is a quote directly from the do's and don'ts section of the seller handbook:
"If someone adds your item or shop to a Favorite List, that is not an invitation to send a Conversation to that person."
Personally, I wouldn't like it if I received a message from a seller just because I favorited something. It would be a bit creepy.
~Kris
PS. Here's the link for the whole Do's and Don'ts article:
https://www.etsy.com/help/article/483#conversations