There are a lot of misconceptions here - Whatever you sell has a value - whether purchased yesterday or a hundred years ago. The selling price, minus that value, and any associated costs is what you would be taxed on, regardless of whether it is £7 or £107, and whether you are a registered business or not.
For example, I am not in business, I am retired, I have numerous paintings that John bought some years ago. They were valued for his estate when he died. If I now sold them for more than that value I would expect to pay tax on the difference less any costs I incurred in selling them.
Each individual transaction is not taken on its own - the total sales, expenditure, and purchases figures in any year from 6th April to 5th April are added up and the profit (or loss) is then calculated.