This is accomplished using the Units of Measure Schedule in inventory setup. For example:
If the main unit is POUND then, POUND = 1 POUND, 1/2 TON = 1000 POUND, TON = 2000 POUND, SQFT = 10 POUND, etc.
The Unit of Measure schedule is assigned in item maintenance. You can then buy and sell in any quantity set up in the unit of measure schedule. So you could buy or sell by the TON, SQFT, 1/2 TON, POUND