What the NS record does, is it tells the DNS system to handball all DNS requests to the Nameserver listed for
*.domain.com
I.E.
127.0.0.1 (A) -- @ domain.com (Root A record for domain.com)
At this point, if you look up
www.domain.com you will see that it will fail!
127.0.0.1 (A) -- @ domain.com (Root A record for domain.com)
domain.com (CNAME) www @ domain.com (CNAME Record for WWW point to domain.com)
Now you can access the site using
www.domain.com.
For each subdomain name, there must be either an A entry, an NS entry (With corresponding A entry) OR a CNAME entry otherwise the subdomain name will point to nothing and will not work.
The difference between NS and A entry is as followed.
NS is the instruction entry to tell the DNS system to handball or pass on all DNS checks and lookups to the name server listed in the NS entry.
A entry tells the DNS system what IP address should be loaded when loading the domain name or subdomain name with the A entry.
CNAME is a pass to entry, it pretty much is a redirection entry, that's why when you load up
www.yoursite.com you get the same page as yoursite.com.
So in turn, YES, using A entries will work for your reseller, however you will need to add these A entries yourself, however, using the NS record to point your domain name to a name server (like BIND) cPanel and most other control panels WILL add this A entry for you.
Easier option is to use name servers, annoying and time consuming option is to use A entries which are manual.