I don't feel safe looking after someone else's stuff.
I can't relax looking after someone else's stuff.
この文脈だと、「落ち着かない」→「To not feel safe, to not be able to relax」
「人のもの」→「Someone else's stuff, someone else's things」
「I don't feel safe looking after someone else's stuff.」、
「I can't relax looking after someone else's stuff.」
どちらでも使えます。
「よかったら郵送しようか？」は、
「Would you like me to send it to you?」
と表現します。
I don't feel good about keeping something that belongs to someone else for a long time.
I don't like holding on to something that's not mine for an extended period.
この場合、次のような言い方ができますよ。
ーI don't feel good about keeping something that belongs to someone else for a long time.
「長い間、誰かの物を持っているのは良い感じがしない」
ーI don't like holding on to something that's not mine for an extended period.
「長期間、自分の物ではない物を持っていたくない」
ご参考まで！
シンガポール 
カナダ 
