Do not return any money or engage with the buyer in any way.

Assuming the buyer hasn’t filed a dispute yet, remove all funds from your Venmo account, and disconnect your bank account. If possible, delete your account entirely.

Venmo, like PayPal, is scum. However, they quickly crumble when faced with any legal challenges. You can also just choose not to deal with them, and they won’t follow up.

Key point: run away while you still have the $$. Make recovering anything their problem, and make it a big problem. This will probably deter them.