EBay may be the fat boy in the collectibles canoe these days, but until they invent virtual fondling, online bidding can’t substitute for actually handling the goods and dickering with the dealer. The 150-plus dealers at Cambridge Antique Market make it the most diverse emporium of old stuff around. China, toys, dolls, and books spill out on the shelves. There’s jewelry, watches, and silver flatware under glass. Imagine the attics of an entire neighborhood gathered in a single building. Who knows what treasures lurk on those five floors?