What do tax authorities think about high numbers of incoming SEPA transfers?

I have never sold for fiat on Bisq so I’m curious, if someone were to make many sales on Bisq and receive numerous SEPA transfers from all over EU to their bank account (along with random character strings as the message) to the point of it being a substantial amount of incoming transfers and/or sum of money, wouldn’t the authorities find that suspicious if they reviewed it? How do you deal with this, do you declare these as (Bitcoin?) sales in your tax forms or what do you say was the reason for the incoming transfer?

I’m guessing that many Bisq traders simply don’t declare it and hope to get away with it (no judgment here), but if one were to come under review it would be difficult claiming you have friends all over EU gifting you money.

Also, whenever I make a SEPA transfer my bank asks me for my reason for transfer. I usually just select “Other” from the dropdown-box of my online bank service. Does anyone have any comment in regards to this?

I think you’d better ask for legal advice next to professionals in your country on the manner to declare bitcoins sold.
As to the tracking of payments, my intuition is that banks are more reactive than proactive. They’ll look into you if they have some previous reason to do so. And if they are amassing bank records and reasons for payments in mass, Bisq’s random digits may raise some brows but I assume loads of people put random strings in their reason of payment.
But if in the future this turns out to be a problem, I think it’d be easier to generate more “normal” strings.

It is somehow like the web portals such as eBay etc. Buying and selling stuff on these web sites also generates bank payments from person to person with some reference number.