What is the "N Ghash/s for sale" though? Are you referring to mining contracts?
No, just that instead of parting out systems entire rigs are sold, in quantity ("everything must go"). So to make it easier, the title for the seller's listing simply refers to the Ghash/s capacity rather than describing the components (e.g., 3X5970).
Here's an example:
What came first was that this is a set of rigs with 2.4 Ghash/s capacity, and secondary was that the 2.4 Ghash is coming from eight 5830s.