You're saying if I sell 1BTC on Bisq and I suddenly get $20,000 via bank transfer, which on other days doesn't happen too often, the bank is going to ask questions?
Yep. It may vary per country, but I'd expect a letter asking many questions (just shy of the color of my underware).
I mean, you could just reply to those questions honestly and say you bought BTC at a certain earlier point in time and sold it online. I don't think it would be a big issue
They'll also ask for a copy of your taxes, including your savings at other banks. Privacy? Lol. If you don't like it, they'll close your account.
it's going to be similar to buying (or selling) another expensive item like a car.
Not really: the car is traceable in your name.
Then, when the day comes that you're selling it all on Bisq, there shouldn't be any issues. Since they can go back and see that you always declared ownership of them and paid your taxes on them.
You'll be fine, as long as you answer all privacy invading questions your bank asks.
Okay; so because I didn't buy via KYC exchange in the first place, they'll ask more questions compared to another sale and may / may not steal my fiat money?
Then I guess a P2P (cash in hand) trade of those BTC might be possible or otherwise a direct sale for BTC. In that case you will be sure not to get anything stolen from you and as long as your tax documents are fine, there won't be issues on that front either.
In general I don't think using a centralized exchange to do something illegal in your country, is going to make it legal. If you follow your laws in the first place, you can pretty surely sell P2P without worries.
From what I've seen, using an established exchange makes the bank "easier" because they know at least the other party isn't a criminal. Now let's say you sell 1 BTC and receive $20,000
tainted dollars P2P. Then what?
That's because the bank has liabilities / duties to the country it operates in and a registered exchange (company) is an easy / no headache source of funds for them, right?