use of en

Bonjour Ann ! In "Marie already bought them all.", we're talking about the specific presents she's bought, hence the "definite" direct object pronoun "les". "Marie en a déjà acheté." means "Marie already bought *some*." Moreover, in that case, you would also lose "all/tous" as you can't say "some all". But here she bought "all of them" (i.e. the ones she meant to buy). I hope that's helpful! À bientôt !