¿Is it necesary creating a Paypal account to pay?
¿Is it posible paying directly by credit card?
Try logging into whatever account you've connected Moodle to, if I remember correctly there's a setting to allow nonlogged in sales. It's usually disabled by default as you loose "PayPal protection" that way.
https://www.paypal.com/uk/cgi-bin/webscr?cmd=xpt/cps/general/OptionalAccount-outside
Check that link, third paragraph from the bottom.
Hope that helps
Ben