Through what reseller account? Obviously you don't mean DirectAdmin; DirectAdmin reseller accounts have no facility for end users to register domain names. Based on your website it appears you're using WHMCS.
There are tjree ways to do this. The first requires custom coding in whatever software you use for your customers to register domains. The second is a lot easier, requires a pair of nameservers used for nothing else, and is in violation of all the RFCs, but it works. The third may or may not be available to you depending on who you use to register your domains.
1) At the same time the user registers the domainyou run code to set up a zone in your nameservers, pointing him to a dedicated IP#. Have a domain set up at at the dedicated IP# (I've set up an example at
http://freeparking.nobaloney.net/) and point the newly registered domain to that IP#. Your site can be a lot more complex than our example, and can use php code to read the domain name the visitor is looking for and even attract google advertising.
2) Create the site as above. Set up two nameservers (or one nameserver with two IP#s). Make these nameservers your
default nameservers for newly registered domains. On these nameservers set up wildcard DNS zones for all the top level domains you sell, pointing to that same dedicated IP#.
3) Some Domain Registrars (mine, which works with WHMCS, for example) allow you to set this up automatically as an option for new domains (but WHMCS may not properly manage this; I'm not sure).
Jeff