No we do not allow oeverselling and we do not recommend it either to a reseller or a potential client.
Overselling is when a web host will oversell the amount of space that the server has available. It is a marketing gimmick which sounds like the buyer is getting a good deal.
A perfect example is a host offering 300 Gb of storage space or unlimited storage space. This means that the host has a server with 300 Gb of disk space and they will host several hundred or thousands of web sites on the server. Each with 300 GB of disk space, hoping that none of them will actually use it.
This is why we cap all of our servers at 150 websites each.