Oh I know its against their T&Cs, then again so is botting, sharing accounts, buying gold and so on, yet we often discuss those here. In fact I recall a thread not long ago on here about sharing an account.
Lets face it, if someone doesnt like levelling and wants to play endgame only, then buying an account is the easiest way to achieve that. Blizzard frown on it but having said that my mate bought his account in 2007 and is still playing it now so them "catching" you is by no means a certainty and on the plus side for Blizzard it gives them 1 more paying subscription that they may of otherwise not got.
Being caught by Blizzard isn't often the issue.
The issue is the person you've bought the account from recalls it (which they can do at any time, there is absolutely NOTHING you can do about that, even if you've moved to a new account and so forth).
A few of the people I used to play with sold their accounts (for quite a lot of money due to the gear the characters had), and then a few months later decided they wanted them back.
The other person was shafted and telling Blizzard gets you nowhere

You can do it by all means, but its a really really bad idea.