limiting new account registrations
JimmyL: If you're really that concerned about multiple users, I think doing the CC check would be the way to go.The larger question I think you should be asking, however, is why this guarantee is so important. You're setting up a huge barrier to new sign-ups - if I came upon a site asking for my CC details, even if it was through PayPal, it would take a hell of a lot for me to move forwards and not lose interest in the service. So you had better make sure that your product is so compelling that it will cause your users to get up from their computers, go get their wallet, and give you their CC number; be warned, this is one hell of a threshold. If you've gone that far, however, I wouldn't bother refunding the money - almost all users who you've engaged enough to get their credit card number won't mind giving you a nickel or so for your services, which you'll probably need to cover the costs of your CC fees anyways. If you do this, I'd 100% go with PayPal - it's easy, and it provides me (the user) with some degree of reassurance that you're not just going to steal my CC number.Even if you do this, however, you still need other checks. I have three different credit cards, all hooked to different addresses and with using different variations of my name. Many banks allow users to generate one-time credit card numbers for online purchases. Multiple people could live at the same address and have cards, and they could even have the same names (like when sons are named after their fathers). Don't forget to analyze how important it is not to have duplicated, or conversely, how much of a problem it would be to have them - as this gets higher, you're going to have to make some tradeoffs (for example you could make a rule that says only one registration per household, and accept that you'll be losing out on users where there is more than one party in the household who wants to use your service).As a point of comparison, the last service that I signed up for that needed this severe of a uniqueness guarantee - the old ICANN@large, anyone? - did a web-based signup, which led to a code getting mailed to you. Additionally, they restricted it to one-signup-per-household, and didn't accept PO boxes.Bottom line is that this is a difficult problem, which generates a very high barrier to entry. So if you've done the analysis and concluded that it's 100% necessary to do this, you may as well embrace it and charge a bit.