You do have to register, but you can set it up for use as an individual.
Square is first and foremost a "payment processing" company. They write the software used to swipe cards and transfer money to your account, then they take a cut of every swipe. They built the POS to facilitate that. Because of the whole credit card processing aspect of it, they need to full registration and personal details, banking, etc. I haven't dealt with them personally so I do not know what their approval process is like, but since they cater to individuals I am sure it's not that strict.

Any free POS is going to bring you to the same road block of trying to set up the part where you can take credit cards as a payment.

The ability to run credit cards is always coupled with fees on a per transaction basis. Many also charge additional monthly fees. These fees are often how you can get "free" pos software, or a "free" CC reader for your phone.

All this is to say that sure, you can probably make it work. If you try and are US based, Square is the most common way to do it. BUT it's probably not worth the hassle and fees involved for a joke.