Lol

I have no idea who should go first

If it's a physical interaction then, there would be no issue

But online? Forget it

On the other note, I think the seller should go first

That is because if he told you he had $60 worth of BTC

Then he sent it over, and due to fluctuations, it appears as $58

If the buyer had send it first, he will be at a $2 lost

So if there is no trust, then the seller should use exchanges for the third party protection