I can't believe someone would use your apartment as a storage unit then take off overseas.
この場合、次のような言い方ができますよ。
ーI can't believe someone would use your apartment as a storage unit then take off overseas.
「人のアパートを物置がわりに使って、海外に行くなんてあぜんとするよ」
to use one's apartment as a storage unit で「人のアパートを物置として使う」
ご参考まで!