Earlier quoted context omitted.
Are there rules against selling to robots? I mean, a great portion of the stock exchange is just that, essentially, no?
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.
The closest thing would probably trades with wild animals. Are there any laws on that? (Probably not, but the situation is not unthinkable.)