A stroll through the packed aisles of Uncle Sam's used-CD bins gives weight to that old adage of one man's trash being another man's treasure. Surely that stack of Steely Dan titles wasn't sold off by the same person who divested himself of a half-dozen Tupac Shakur CDs? No matter. Don't waste time pondering the sociology behind the Beach's musical tribes as their respective tastes flow back and forth at $7.99 a pop -- just get a' hunting. There's something here for everyone, from the latest hip-hop releases to the freshest trance straight off the boat from Europe to old-fashioned rock and roll, complete with big hair. And best of all -- in a marked departure from most area music shops -- every CD can be previewed to your heart's content. But please remember as you strap on a pair of headphones: Leave the singing along for your home shower routine. The staff at Uncle Sam's may be friendly and helpful, but they still have limits.