This product is handmade by the artisan one by one.
This product is not mass produced. The artist makes them by hand one at a time.
この場合、次のような言い方ができますよ。
ーThis product is handmade by the artisan one by one.
「この商品は職人により１つ１つ手作りされている」
ーThis product is not mass produced. The artist makes them by hand one at a time.
「この商品は大量生産ではなく、職人が１つずつ手作りしている」
ご参考まで！
カナダ 
