As you say.. how sustainable are they??
Im not sure how much is a rip-off at other sites? No idea what the mark-up is on new games.
Just make sure that you use a credit card when you order. That way, if they do go bust (like shopto's last company did) then you wont lose your money.
Always buy online with a credit card. It covers you from any problems
shopto are generally really good, I ordered rainbow 6 vegas for the 360 on tuesday for £30 and it come wednesday morning and if you have any queries then contact then as I find them very helpful and they normally respond within half an hour max.