Everyone else has good advice. If they’re real it’s a good deal. This is hard to do but I would try to verify who they are and observe the way they interact with you. If they’re in a rush to sell the account and resist when you ask for more information that would be a red flag.

I’m not sure if paypal protection covers this, but it wouldn’t hurt to do the transaction through there and accurately describe what you’re paying for on the invoice, ie: “full ownership of instagram account @______ with x amount of real followers”. Even if paypal doesn’t explicitly protect you, again someone trying to scam you is going to be less cooperative in my opinion.