It is highly not recommended to buy cheap office keys elsewhere, except from Microsoft. These keys are typically gathered from MSDN channels and a single key are sold multiple times to many users (Read more), which may lead to deactivation due to over-activation. Never buy keys from grey-markets which essentially have no legitimate support or license and still keeping them in business. You might as well do the following method.
![](http://i.imgur.com/s86LgSu.png)