Overselling means allocating more capacity across your packages than your reseller plan actually holds, on the assumption most clients will not use their full quota.
WHY IT IS COMMON
Most websites use a fraction of their quota. A 10GB allocation across twenty accounts on a 50GB plan usually works, because the average account uses perhaps 1GB.
THE RISK
It works until it does not. If several clients grow at once, or one starts storing video, you hit your allocation and every account suffers. Uploads fail, email stops being delivered, and you are dealing with twenty angry clients simultaneously.
IF YOU DO IT
Do it deliberately, with a ratio you have thought about, not by accident. Monitor total usage continuously, not occasionally. Keep genuine headroom, and act when you reach it rather than when you exceed it. Have an upgrade path ready and budgeted. Never oversell CPU-bound resources on the assumption disk behaves the same way.
THE ALTERNATIVE
Sell what you hold. Fewer accounts, higher prices, no capacity crises, and a story you can tell honestly to clients who ask. For a small reseller building a reputation, that is frequently the better business.
