You know the story — the landlord raises the rent, and a favorite local store is forced to close. The St. Marks Bookstore has been a popular local institution for decades, important both to residents and college students attending nearby NYU and Cooper Union, and managed to stay open even as chains and online shopping snagged customers. But the neighborhood around Cooper Square is getting gentrified, and the landlord wants to raise the rent on the shop at 31 Third Ave. You can help keep the St. Marks Bookstore open — click here to sign the petition to encourage the landlord to give the bookstore a break. Who is the landlord? Cooper Union.