Record collecting started out as a hobby for Jean Haffner and ended up as a career. The owner of the Record Exchange grew up in the Texas Panhandle town of Canyon, where his next-door neighbor was Buddy Knox, the rockabilly artist who recorded the hit single "Party Doll." Haffner never got over the infatuation with rock & roll that he acquired as a teenager. His record collecting became a full-time occupation for him 25 years ago. His store, which is now located in a huge former public-library building at the corner of Eichelberger and Hampton, deals exclusively in used records in every conceivable format: cassettes, LPs, CDs, even eight-track tapes. He says he offers one of the largest inventories in the country, with the used CDs a particular bargain. Haffner says he decided to devote his whole life to his musical interests after the death of Elvis Presley in 1977. So the giant used-record store in South St. Louis is sort of a tribute to the King.