If you are an enom reseller you use their merchant facilities when you use the registry rocket and pdq solutions enom has. You can use your own but that requires using the enom API and your own custom API for credit cards.
A master account pays out the full fee (ie 500 or whatever up to 7g) to enom to get the bulk discount and should have a direct line to enom for support. Also their pricing will not change overnight. A reseller under a master or other reseller account has no control over their pricing, their support should come from their upline and they need only $100 to get in and start buying domains, well less if by cheque etc but cc is too easy nowadays.
Basically advantages of Master is it gives price control and direct support while reseller is cheaper to start.
Both can have sub-resellers and use all of enom's solutions.
Hope that helped you a bit.
