The text plays on the common saying 'Old is Gold', which means that something old is valued or precious. However, it adds a twist with 'but never sold', creating a rhyme and a contrast without relying on different meanings of the same word or similar-sounding words. The humor comes from the rhyme and the contradiction of something valuable ('Gold') that is not sold, but there is no linguistic ambiguity.
Old is Gold , but never sold .