5. A method, comprising: retrieving, by a processor, a plurality of advertisements from multiple content sources; identifying, by the processor, keywords of the plurality of advertisements; determining, by the processor, similarity rankings of the advertisements based at least in part on the keywords; selecting, by the processor, a keyword based at least in part on the similarity rankings; and purchasing, by the processor, the keyword from a 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 a buyer.
5. A method, comprising: retrieving, by a processor, a plurality of advertisements from multiple content sources; identifying, by the processor, keywords of the plurality of advertisements; determining, by the processor, similarity rankings of the advertisements based at least in part on the keywords; selecting, by the processor, a keyword based at least in part on the similarity rankings; and purchasing, by the processor, the keyword from a 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 a buyer. 10. The method of claim 5 , further comprising: determining relatedness rankings of the keywords based on the advertisements.