"The merchant buys ten cookies."

Translation: Mercator decem crustula emit.

[lmrs1]

Should this be 'mercator decem crustulaM emit'?

[Moopish]

crustula is the accusative plural here (the form is also used for the nominative and vocative plural). crustulum is a second declension neuter and does not have a form that looks like crustulam (which looks like the first declension accusative singular).