What do you get for the person who has everything? How about an 8-track tape? You can pick one up, along with more modern items, this Sunday at the Somerville Rock And Roll Yard Sale. Some of New England’s top record sellers will be on hand with mountains of vinyl, cassettes, CDs, audio gear, music memorabilia, and other interesting items for you to peruse. The Rock and Roll Yard Sale runs from 11 a.m. to 5 p.m. in the Union Square Plaza this Sunday, Sept. 7.