I think I've already replied to your email on this topic, but here's my reply again:
Using Redirection is not a reliable method to pass order data back to your system for the purpose you describe (registering the buyer for a user account in your system), since we cannot force every third-party payment processor's checkout site to automatically redirect all buyers to any thank-you page URL after checkout. E.g., PayPal only provides a Complete Purchase button the buyer must click to proceed to a thank-you page. If the buyer has no incentive (such as claiming a download they'd paid for) to click through to visit that page at some point, they might never visit any thank-you page at all, so the GET variables would never be handed-off to your page. The sole purpose of the GET variables we append to your URL when redirecting is simply so your page scripting can grab those variables to customize the page content displayed to the buyer, that's all.
To reliably transmit order data to your system, you should instead be using our custom/3rd-party Integration feature, which is designed to mimic PayPal IPN as closely as possible, so any ready-made off-the-shelf script or scripting approach which can handle PayPal IPN should be able to handle our Integration submissions with little or no modification: