Aftermarket also referred to as Secondary Market represents the main market for already registered domain names, which are made available for sale. Once the current registrant decides to sell the domain name, this is possible only on the aftermarket.
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. 
The aftermarket may also refer to any type of market where purchases of already owned products are traded. In order to sell and purchase domain names on the aftermarket the registrant usually uses a third party website such as SnapNames (division of Oversee.net) or Afternic. 
Furthermore, lately the keyword domains are getting more valuable, and their main purpose is to drive traffic to a company's website by means of various ways. The aftermarket fluctuates a lot and this is not good. Usually, huge prices are being paid for domain names raising the offers of domain sellers and their expectations. But in the same time there are situations when purchasing domain names in the corporate sector leads to relatively low prices. These fluctuations influence the demand and supply on the aftermarket of domain names. 
ICANN and the Aftermarket
According to ICANN's meetings, it seems that the secondary market is transforming into a primary market because this the main method through which domain names are being sold and purchased. On the aftermarket the domain names are being traded at increased prices and the ROI continue to increase due to the competition.
Recent Aftermarket developments
Lately, when it comes to the secondary market of domain names, the main drive is represented by the Internet advertisement on search engines. The increased number of registrations on the secondary market for domain name and for targeted Internet search advertising have led to the development of a wide variety of new practices and business models on the Internet. The domain name speculation is being replaced by more sophisticated and complex methods for establishing the value of selling, purchasing or even leasing the already existent domain names in the secondary market. The main purpose of domain names is enable traffic attraction from search engines and generate cost-per-click advertising revenue. Nevertheless, the aftermarket is definitely becoming a very competitive market and maybe even industry.
The aftermarket is far more complex than it seems. Registrants should not let the domain names to expire without either using them to earn profit or selling the domain names to reach break-even point. The competition on the aftermarket also influences the registries and may lead to various technical demands for registries.