There is a mansion that has been for sale for over a decade in the town I live in.
There is a huge house that has been on the market for over ten years in my town.
a mansion なので be動詞は are ではなく is を使ってあげましょう。
a decade で「10年間」なので「10年も」と言うなら複数形にする必要はありません。
ーThere is a mansion that has been for sale for over a decade in the town I live in.
「私の街には10年も売り出されている豪邸がある」
ーThere is a huge house that has been on the market for over ten years in my town.
「私の街には10年も売りに出されている大きな家がある」
to be on the market で「売りに出ている」
ご参考まで！
"There is a mansion in my town that has been on the market for ten years."
"There is a mansion in my town that has been on the market for ten years."
"There is a mansion in my town that has been for sale for ten years."
"A mansion in my town has been on the market for ten years."
自然で英語らしいのは
There is a mansion in my town that has been on the market for ten years.
です。
意味は
「私の街には、10年間も売りに出されている豪邸があります。」
です。
カナダ 
アメリカ合衆国 
