Can someone please explain this buying keywords term?

The phrase "buying keywords" doesn't mean you pay for keywords.

It's a term for keywords that indicate the searcher is likely to buy. "Buying keywords" are different than "research keywords". Think about the commercial intent of the searcher...if they have the intend to spend money, they search with "buying keywords"...if they are just browsing, they search with "research keywords".

For example, these would be "buying keywords":

price for nikon d camera
cheap nikon d cameras
how much does a nikon d camera cost
nikon d camera on sale
nikon d camera for sale


Whereas "research keywords" might be:

"nikon d accessories"
"nikon d instruction manual"
"how to take black and white pictures with a nikon d"
"cleaning a nikon d camera"


You can see that the first set of keywords would be used by someone ready to spend money. The second set is probably someone just looking for information.

The upshot is that buying terms "convert" better, mean they are more likely to turn into ad clicks or sales.

Does that help?

Ghalt hit the nail on the head above with 'buyer' or 'buying' keywords, but as far as literally buying keywords is concerned, some folks do sell lists of potentially profitable keywords that they have spent time researching and finding.