He's not selling the actual memorabilia (as others have pointed out) - he is actually selling them as NFTs (non-fungible tokens). Essentially, you are 'buying' a digital form of the artwork, but in many cases, the actual artist will retain part of, or all of, the copyright.
The saying, 'a fool and his money are soon parted' certainly does come to mind here, but some NFTs have sold for staggering sums. An NFT of an animated GIF of a flying cat which became a popular meme sold last year for more than half-a-million dollars, which is astounding for something you can't physically own. Many experts reckon the NFT bubble will burst sooner than later, which is probably why people like Julian Lennon are trying to cash in on it while the going is good.