Older stores are going on the sales block at an accelerated pace. This week 41-year-old Owl & Turtle Bookshop in Camden, Maine, announced that it, too, is for sale. After being owned by three families, the owners are looking for a fourth to buy the one-time bookstore/B&B, which expanded in 2003 and moved to Knox Mill Center just off Main Street. Named this year’s Best Bookstore in Maine as part of Down East magazine’s Readers’ Choice Awards, the Owl & Turtle carries 10,000 titles and has a strong selection of marine titles and children’s books. The sale is being brokered by Ken Twaddel at Camden Real Estate Company.