What is the strongest liquor sold in the U.S?

It depends on the state. Some states permit the sale of 190 proof (95% alcohol by volume) neutral spirits such as Everclear. In other states, the highest ABV percentage allowed is lower (for example: 75% in West Virginia, 60% in California).

It's rare to see anything stronger than 95%, because getting there requires special techniques; 95% ABV is what's called a "azeotrope" and is the highest concentration of alcohol that can be produced just by distilling a mixture of ethanol and water. Companies that, for some reason, require a higher concentration of ethanol can purchase what's called "absolute alcohol" which is very nearly pure ethanol (you don't want to drink it, though; in addition to being expensive, it generally contains small but measurable amounts of the carcinogen benzene).