Overselling basically means to sell beyond the means of delivery. It keeps hosting prices at low rates. And it is also widely abused as many companies go too far with it to keep prices lower than the competition. Overselling is also common in other industries such as the telecommunication sector. As the name indicates, it means you sell more resources than there are. An example: the $45.95 Reseller Plan 1 comes with 40,000 MB of space and a whopping 200 Gig of bandwidth. As a 123 reseller you are now able to create domain hosting accounts with very high or even UNLIMITED space and bandwidth allocations e.g., you could create 100 hosting plans with 1000 MB space and 10 GB bandwidth each. Experience shows that 95% of customers do not even use 30 MB, yet many customers want a lot of space and bandwidth. Just look at your own existing plans. Therefore, rather than filling up only 10% of the space and bandwidth you bought you will now be able to utilize your account to maybe 90% and make a much larger profit. Even if you don't resell hosting accounts, you still have the advantage of being able to set all your domain accounts to "unlimited" and thus do not need to worry about joggling around with the allocated space/bandwidth.
Overselling is only available for Reseller Plans #1, #2, and #3.
123 Reseller Web Hosting Plans