The only thing that's changed is the reporting requirement by the selling platforms. The rules have broadly been the same ever since I have been in the accountancy profession (more than 50 years).
There's a handy concept all accountants have to learn when studying, which HMRC have kindly codified
here. That's the badges of trade, ie what factors come into play when deciding whether transactions constitute a trade or not.
The mnemonic when I was studying was SO FIRM FAST:
Subject matter
Ownership period
Frequency of transactions
Improvements made to the goods between purchase and sale
Reason for sale
Motive for purchase and subsequent sale
Finance source
Acquisition date, reason and costs
Similar Transactions comparison (I always think it's a bit of a cheat in a mnemonic for two initials to appear in one line, but there it is).
Now this is a balance of probabilities thing, so just because you sell ten guitars in a week doesn't necessarily indicate a trade, but to sell one a week for a year might, particularly if you didn't play them for a while in between purchase and sale. But the motive is particularly key here. If you regularly buy guitars knowing they are less than market value with the main intention to flog them at a higher price, particularly if you do some work on them to enhance their value, then it's likely you are trading.
If you
ARE trading you have a responsibility a) to notify HMRC within three months of starting trading and b)if you make a profit, then you must self assess the income tax and report it to HMRC annually.
I know a lot of us perform and are sometimes paid. Although that's broadly covered by the same rules as buying and selling instruments, there are some additional things HMRC tend to look at.
In either case, you can deduct all sorts of expenses from the Gross Profit before you calculate the tax.
In certain cases there may be Capital Gains Tax implications from buying and selling immensely expensive instruments, but the sale of most of our guitars would be covered under the general exemption for chattels.
This is all very rushed (it's January - eek!) and generic. If you think there may be an issue for you, you should consult an accountant specialising in tax. You can often have an initial half hour consultation for free, and the accounant can give you specific advice relating to your own situation.
By the way, full disclosure, I did not remember the mnemonic of the badges of trade from 50 years ago, I had to look it up (but I
DID know where to look!)