Are the resellers customers different from the regular customers? In other words, can a regular customer also be a reseller's customer, and can a customer change from regular to reseller?
Seems like the easiest would be a separate table for resellers customers. But, what else does this database do? Are you tracking interactions, sales, etc.? What kind of reports do you need to generate?
The only difference between a regular customer and a re-seller is that the re-sellers have sub customers and regular do not have sub customers. From a billing perspective if you are a regular customer you get a bill, for re-sellers they get one bill for all of their sub customers.
I am using this DB to Track customer activity, trends, billing details for a very complicated Billing system so that it can be eventually be simplify. I started it in a spread sheet but it has quickly outgrown it.
If the data recorded for the customers that you bill and the data that you record for the re-seller's customers is mostly the same, I'd use a single table and set up a self join (a relationship linking two Tutorial: What are Table Occurrences? of the same table) to link a re-seller to their customers.