I am no stranger to buying online, I do tend to be as careful as possible mind you.
I recently made a credit card purchase online, and when asked for the 3 digit number on the back of the card came over a bit confused, and entered the 3 figure number from a completely different card of mine.
The resulting security? Nothing. Order taken, payment accepted, product supplied.
These 3 figure security numbers are very good protection against online fraud, are they not