make sure when people fill out their details, that they recieve a receipt and that they get their license key. Instead of not being able to purchase it.
I found that i could not purchase the product, but all my details and payment method was logged.
I think this is a very serious flaw it the sales of it. If people can't obtain the product without waiting for problem to be fixed they are not going to be very impressed.
I looked on the forums and saw atleast two other instances of people that had the same thing happen.
best thing is that you log their details into a database AFTER they verify that they actually have the license key. Not create their account before they have purchased something. Otherwise you are unfairly taking their information.