I’ve been buying and selling instagram accounts for while now and the best sites are EPICNPC and PLAYERUP.

https://www.epicnpc.com/forums/instagram-accounts.588/
https://www.playerup.com/accounts/instagramaccount/

They are both forums with marketplaces to buy and sell instagram accounts from PVA to established IG accounts with 1,000,000+ followers.

I highly recommend whether you buy/sell to always use their middleman services for all of your deals.