The difference between Overselling and Unlimited Resellers:
The difference between Overselling and Unlimited is with the Overselling you are limited to the number of domains you are allowed, if you were on the Bronze Overselling package and you have 20 domains on your reseller plan and you gave them each 500MB of bandwidth - your limit of 10GB would be finished, but if in the month they only use 300MB of there 500MB allocation than only the 300MB gets added up, which will mean that you are left with 4000MB so you can add more resellers if you want. The Overselling the reseller package stays the same every month - R99 with your set web space and bandwidth/traffic, but what is counted for the clients is how much they use in the month and not what they were allocated, so if they use less than allocated it allows the reseller to get more domains.
With the Unlimited reseller package, you are not limited to the amount of domains you have, but the amount of bandwidth and web space that is allocated, is what is counted at the end of the month, so if you give your client 500MB and they only use 250MB, at the end of the month the 500MB will still be added together.
So with the Overselling you are limited be the number of domains, where the Unlimited you are limited by your bandwidth and webspace allocations.