Robots can't own property, as they are not recognized as a person.

You may sell to someone represented by a robot, but you're not selling to the robot.

When using a vending machine, your making a trade with the vending machine company, not the machine itself.

Hm, so selling to a robot without someone behind to represent would be extralegal I guess, and if any problems occur during the trade you'd be on your own?

The closest thing would probably trades with wild animals. Are there any laws on that? (Probably not, but the situation is not unthinkable.)

Can a robot be the sole asset, employee, and officer of a corporation?

A robot cannot be an employee or officer of a corporation, you need to be a person for that.