How much of a problem is it? Well, enough for i-to-i to give this very strong but unconvincing warning about the consequences of trying to do so:
The REAL consequences of cheating on your TEFL (LINK BROKEN removed 5 March 2017)
Given that, how much can employers trust purely online (or even some blended) TEFL certs?
And some idiot publically offering to pay someone to finish theirs here:
http://www.boards.ie/vbulletin/showthread.php?t=2056813727