Does anyone know a good app on which I can sell tack in Canada Ontario ?

TackHunter or instagram!!

You can sell stuff on Instagram

TackHunter

It's not an app but try equistation!

I sold my old saddle on Kijiji

I think you can return used tack to a store called toudor tack, but I'm not sure if they have it in Ontario, I live in Alberta.

Equine now