Depends on the country, but in Slovakia, if the unit is the same, then it's not selling, just changing form. That is one of the best use-cases for Monero in entrepreneurship. Say you want to have a Bitcoin reserve in a company, but also want to pay suppliers with some crypto. Sale is a taxable event that goes to weighted average and triggers capital gains. So actually buying Monero and spending it at the same moment is a tax free event (buy and sale price are the same, minus the fees), but doing it with anything Bitcoin (does not matter if Cashu/fedi/lightning) would be taxable event. Of course you can also use stablecoins.

You mean like doing a swap from bitcoin to monero and having that monero being send to the person you are paying? Doesn't that count as a sale and trigger cap gains?