Jason Ridpath's short poem is about the odd gap between what our experiences mean to us and what the world says they are worth. The opening line insists that the things that enrich a life can't be sold, because their value is too great. The second line turns this around: because those experiences are personal, other people see no worth in them and wouldn't want to own them. So the same thing is priceless and worthless at once, depending on who is looking.
The third line, "On the shelf it sits growing old," gives the idea a quiet, sad image. A memory or experience that can't be traded just sits there, unclaimed and slowly fading. The final line is a direct question, and the speaker sounds frustrated and a little weary: why do we still judge people and things by "weights of gold"? The mood is reflective and wistful, with an undertone of protest against a world that only counts what can be priced.