Unless you own your own server, overselling probably happens in some way. Lets say if Company A resells a server with 40GB Disk space to Affiliate A,B,C,D,E,F,G,H,I, J and offers each one 6GB Disk space. Company A just oversold 20GB of disk space, but thats not all that is oversold. Lets say the Affiliates each oversell 10 accounts, claiming each account receives 1GB Disk space. That would mean a total of 100GB was sold, and only 40GB is actually there. Now if each account only uses 35% of its alloted disk space, 35GB of the disk space would be used which is EXTREMELY dangerous (only 5GB to spare). That is 250% overselling even though the Affiliates never really think about other Affiliates overselling or even Company A is overselling. My point is overselling is very dangerous unless you own your own server.

Or maybe, if you own a server you shouldn't oversell reseller accounts :)