the RO membrane (which is the expensive part) is probably dried up.
also its not DI, just RO.
you dont need credit card, you could probably pay-pal also. and even with credit card, if the guy doesnt send it or whatever, you can always stop payment or dispute it (especially if you use Amex).
but i understand where you are coming from, i'm like you, i dont like buying anything from total strangers, you hear too many stories of people that get scammed. unfortunatly its hard to blindly trust random people. but at least the nice thing about MR is that at least we all know each other for the most part and can trust each other. but someone across the country, i wouldnt be able to trust.