Nobody is going to accept this as payment for something illicit. They know you've seen the private key. They have no reason not to assume you have something set up to sweep the wallet as soon as they accept the "payment."

“Opendime is a small USB stick that allows you to spend Bitcoin like a dollar bill. Pass it along multiple times. Connect to any USB to check balance. Unseal anytime to spend online. Trust no one.”

https://opendime.com/

That's not the same as just "handing someone your private key."

It involves either buying or creating a tamper resistant device and a reputation for authenticity.

It's significantly more complicated.

You can put the money in an off chain escrow like Lightning for things like this though.