piffling

Hello I've been told by my son he is looking to sell the Lynx he bought many years ago at a car boot sale and use the money to get an Atari computer as he's started programming and wants something to play around on. He's also just got a promotion & payrise at his work, and is now earning just under taxable territory (according to this wage tool). I'm worried because the profit he will make on the sale if it will end up being taxable? Not quite sure how it works as we generally donate old stuff rather than sell it. can someone put my mind to rest? Thanks in advance