。。
It's no use starting a bento shop — they're everywhere.
Literal
Bento-shop [obj-を] started-even useless-よ. Commonly exist [because-から]-ね.
Several casual markers. (1) 弁当屋さん adds さん to the shop name — a polite/familiar way to refer to shops and their owners. (2) ~たって is the casual/colloquial contraction of ~ても ('even if'). (3) むだ ('useless, wasted') as a standalone predicate. (4) ざら ('commonplace, not rare') is an informal adjective/adverb — ざらにある = 'commonly exist, are nothing special.' (5) からね provides a reason with a seeking-agreement flavor.