Highly recommend anyone making a true PROFIT from this business talk to a good tax lawyer in your own state. I used to write off expenses in a high-end horse breeding project in which the difference between a hobby and a taxable business was determined by whether you could declare a PROFIT in 2 out of 7 years.
So by all means, you must document your expenses (Roon, Qobuz, postage, Absolute Sound, packaging, internet fees, cable fees, any car use for the business, travel to shows, meals at shows, etc., etc. It's pretty easy to show a loss so you can write off stuff but your sphincter can get a workout around year 6 if you haven't declared a profit yet.
If you sell a pair of old JBL's for $200 more than you paid for them, nobody's going to know or care, but if you're making a living, it's taxable like any other job.