Has anybody used Web Form Builder as a shopping cart?

I think it has been at least attempted quite some time ago. But If Eric comes along here, he will probably be able to give a better answer.

Form Builder does have payment options built in. I wouldn't try to alter it to make it behave like a shopping cart as that takes some very robust scripts to keep track of items and take payments.

I have never used web form builder as a shopping cart but do use it to collect information before transferring people to a seperate page where I have paypal setup for purchases. I would not continue building webforms using FormBuilder until it gets updated as most webhost providers have or are upgrading to PHP 8.2 and the old FormBuilder doesn't work with that version. So building with it now will just set you up to fail.