but 99% of people here aren't running the business.
I get where you're coming from though, there is the risk that this move could end up with hobbyists in the crossfire, or at that price anyone selling a secondhand car or posh push bike would be reportable.
buying secondhand stuff I don't have receipts for how much I paid or anything - but the taxman would have to show that you were selling your bitsas for profit, not that you're selling lots of nice bitsas - and the reality is if you set your music playing up as a self employed side hustle the costs of storage, strings, insurance, new pedals, depreciation etc would soon offset any potential profits you might be making.