When you are a domain name reseller, your clients register domain names using the service accessed through your domain name reseller account. The domain name company bills you, and you bill your clients. The domain sellers from whom you buy domains usually provide all the lookup tools and billing software necessary to setup this business. They also provide API’s so that everything can be integrated in your website.
You basically buy domains at bulk prices and retail them to your clients by adding your profit margin. Some of the registrars that offer reseller options are BulkRegister, eNom, Tucows, and Wild West Domains