Can you answer my question about PayPal?

I live in the U.S. I signed up for a PayPal account using a nearly-empty prepaid AmEx gift card. I figured since it can be used in stores as a credit card, it would work online. It did. So now Im signing up for this online sales website(where I would sell things for profit), and I'm wondering. What happens when they transfer money to my paypal? Will they try to put money on the card and have an issue? Could the card limit the money to the 100$ because its was a 100$ gift card? I dont have any actual credit cards. Will I have to get one in order to properly use PayPal? Or is PayPal solely an online thing that wont even involve the card? HELP!!!