Tradesy, Instagram, The Real Real (I've never used before but I see MG bags there)
I like Tradsey a lot because you have a lot of control over the price/description and people can "make you an offer" if they think the price is too high. My guess is that your bag would sell pretty fast.
Thanks ladies! I checked out tradesy since I didn't have any luck with mercari or poshmark. It ended up getting sold the same day I listed it!