Depends ,but some Directi and eNOM resellers offer sub-reseller arrangements for free ,(Including Namepros ) ,but this often involves some component of prepaying a balance into a drawdown account to be used for purchases.
and there are other Registrars that offer free reseller accounts like
the Australian Examples Below
Similiar available in most countries
Bottledomains.net.au (W'sale Buy Price .com's $8usd)
Netregistry.com (W'sale Buy price .com's $12.40USD)
But usually with most of the free reseller accounts ,your buy prices for .com and other gTLD's are very expensive and impossible to sell
You only other alternatives is affiliation with an existing registrar or reseller . This is where you refer your client or buy on their behalf at a domain registrar site and they pay you a commision for the sale.
Most major registrars have some form of Affiliation program
If you are only planning on small volume ,this is often a better solution.
One additional comment ,having a RESELLER agreement will not
make my site a domain registrar?
. To become a Registrar involves a lot more than any free or Paid reseller agreement.