This post is the definition of “caveat emptor,” or is it simply buyer’s remorse. In any case, the OP is expecting too much from a simple stereo transaction. We all have similar stories, but to answer your question, if the piece functions well then there is no additional guarantee. Things break, but how can the seller predict when that will happen? As far as allowing returns, unless specified in advance, nothing should be assumed to be returnable. Move on!