4. A method, comprising: retrieving, using at least one computing device, over a network, a plurality of content items from a plurality of content sources; identifying, using the at least one computing device, a plurality of advertisements embedded in the plurality of content items; identifying, using the at least one computing device, keywords of the plurality of advertisements; determining, sing the at least one computing device, similarity rankings of the plurality of advertisements based at least in part on the keywords; selecting, using the at least one computing device, a keyword based at least in part on the similarity rankings, where a historical price from a keyword seller for the keyword is less, by a defined amount, than a historical price from a click buyer; and purchasing, using the at least one computing device, the keyword from the keyword seller for placement of a first linked advertisement, the first linked advertisement configured to be selected to cause the presentation of a second advertisement of the click buyer.
4. A method, comprising: retrieving, using at least one computing device, over a network, a plurality of content items from a plurality of content sources; identifying, using the at least one computing device, a plurality of advertisements embedded in the plurality of content items; identifying, using the at least one computing device, keywords of the plurality of advertisements; determining, sing the at least one computing device, similarity rankings of the plurality of advertisements based at least in part on the keywords; selecting, using the at least one computing device, a keyword based at least in part on the similarity rankings, where a historical price from a keyword seller for the keyword is less, by a defined amount, than a historical price from a click buyer; and purchasing, using the at least one computing device, the keyword from the keyword seller for placement of a first linked advertisement, the first linked advertisement configured to be selected to cause the presentation of a second advertisement of the click buyer. 5. The method of claim 4 , further comprising: presenting relationships between advertisements across websites.