The cheaper, and easier way, if you're looking to start selling domains with a lower barrier to entry (but less control over how much you pay/how you sell your domains) is to find a white-label reseller registrar.
The first aspect is that every* TLD has it own registry and system. For the generic ones you got ICANN accreditation process, but there is also a bunch of registrar reseller that act as a middle man between ICANN and other registrars.
Usually most processes involve some form of capital investment and/or technical capability. Country specific TLD can either be easier or much much harder depending on which country.
https://www.icann.org/resources/pages/accreditation-2012-02-...
The cheaper, and easier way, if you're looking to start selling domains with a lower barrier to entry (but less control over how much you pay/how you sell your domains) is to find a white-label reseller registrar.