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I don't know of any SEs that just outright sell keywords. There are those like Goto that take bids on keywords and then charge for click thrus from searches on those words. Essentially the person that bid the highest gets the #1 spot and so on down the line.
Not having a any budget for pay-per-click engines I have never fully explored them. (I am sure though that there are others that will be glad to add to this thread...hint..hint;))