How would an amount of antimatter be measured for sale?

By the amount of matter displaced.

Antimatter has positive mass. You measure it just as you would regular matter.

I would imagine it would be sold in non-kgs, except in the US where you would purchase it by the non-ozs.

In most sci-fi I’ve read, it’s still measured in grams or (less often) kilograms. However, current technology is only capable of producing a few atoms at a time, so we currently measure anti-hydrogen quantities in atoms, especially since it’d take ~6.02*10^23 atoms to make a gram.