i lol'd
that's more than a naive miscalculation ...
1) You not only need a processor, but you need a server, bandwidth, script, backup solution - depending on the revenue of the store this percentage varies of course
2) People KNOW + TRUST sites likes C4S, YooGirls, Fetishvideos and others even if they do not know YOU => you'll see conversion 2-3x times better on this sites than sending to your own
3) You've nearly nothing to support, there are always people that don't know how to order/download/etc. - but that's not your problem as it's not your site - the site will handle this support - the only customers you've to deal with are the ones sending requests, etc.
4) Traffic for all kinds of (micro-)niches is usually there - try to get a good amount of high quality (micro-)niche traffic and you'll see how important that is for small stores
So ... if that's all so fucking easy - why don't you start a competitive site? Could earn you huge money if it's easy done - right?