Care to say where are you from? I would like to know how it goes for you, keep us posted. It would be useful to know what amount are you selling. I know its none of your business but its really relevant to know what to expect depending on which amounts are being dealt with. Typically the higher the amount the deeper the audit I would guess.
Another point that could be made is.. assuming you present screenshots of the payments for the forum posts, couldn't they just know your entire earnings? they would know your forum account and thus figure the entire history, so you would need to sell the entire stack? What if I just don't want to and want to keep it and only sell a small %? All these things keep me up at night, I just don't know what to do to be honest.
The conclusion thus far for me is that governments have no idea what they are doing neither do accountants, so there's really nowhere to go in terms of safety, other than not doing anything and deciding what to do in the future.
I report it as an income. It was the best thing to do in my case. I preferred to think large, so even if I'm wrong, in the worst case they will have to pay me money as a refund. I won't be blamed for trying to hide an income. Which is getting more and more difficult in France anyway, especially since now they use big data to hunt down fraud and all that.
The good thing is I am paying about 6% only (instead of ~35%). and to be safe I keep track of every trade I do with it ref # etc. since the govt now ask us to report every crypto exchange account (or a fine of 750€ if I'm correct)