I dont think if there's a legal way of selling an Instagram account, though we know that there's no law prohibiting selling social media accounts it might no one will sell genuine social media accounts which considering you also sell your personal information.
I have doubt if there's someone who wants to sell their social media account with huge followers most likely you're buying a number of followers but not actually the real users. It's most likely a bot or inactive account. So it's not really worth it if you want to target an audience.
Besides, the
Terms of Use of Instagram stated that.
You can’t sell, license, or purchase any account or data obtained from us or our Service.
This includes attempts to buy, sell, or transfer any aspect of your account (including your username); solicit, collect, or use login credentials or badges of other users; or request or collect Instagram usernames, passwords, or misappropriate access tokens.
So I think anyone who wanted to buy an account must be careful, it seems you throwing your money away.