Humana Second Hand is a great fairly large chain of 2nd hand stores (all round Berlin) , they have two stores in Neukölln:
Karl Marx Str. 13, 12043 Berlin
Karl Marx Str. 125, 12043 Berlin
The best store for digging is the Hackescher Mkt one though, no hipsters taking all the best clothes, and a slightly more upmarket hood means more gems.