The Aftermarket, which is also referred to as the Secondary Market, is the market for already registered domain names which are made available for acquisition.
The secondary market supposes the purchase and selling of domain names after registration, at amounts larger than the typical fee that a registrar would charge to an individual registrant. 
In order to sell and purchase domain names on the aftermarket the registrant usually uses a third party website. According to ICANN's meetings, it seems that the secondary market is transforming into a type of primary market as it has become the main method through which domain names are sold and purchased. On the aftermarket domain names are being traded at increased prices due to the competition.
Recent Aftermarket Developments
The increased number of registrations on the secondary market for targeted Internet search advertising has led to the development of a wide variety of new practices and business models on the Internet. Domain name speculation is being replaced by more complex methods for establishing the value of selling, purchasing, or even leasing the already existent domain names in the secondary market. Thus, the aftermarket is becoming increasingly competitive, and arguably becoming an industry unto itself.