In this case, you are going to need to explain the origin of your coins, and im not sure how I would explain I made some BTC posting in some internet forum, this may not be convincing to them even if it's the truth.
But it is the truth.
You should hopefully still be able to prove that you own the account that posted. You'll be able to point to the thread where you signed up. The thread will be there that describes the sig campaign and the pay rates. If you're sensible you'll retain the private key of the address that received the payments and lists each tx.
Screenshot everything now just in case.
People deposit millions in banks from selling art made out of their own sperm and turds. This is less weird by a long way and fully traceable.
Not less weird at all, it's very weird to them, and precisely because it's traceable, you must prove it, if you can't prove it, then how are you supposed to sell?
Again, suppose that I sent coins into Mintpal, traded 1 BTC for LTC, and sold this LTC when it went up for a profit of 4 BTC, I end up with 5 BTC, then I deposit this 5 BTC back into my wallet.
Mintpal dies out of nowhere and now you don't have your account anymore. How do you prove the origin? and if you can't prove the origin, you are fucked. This is why im scared to sell BTC. Yes you may be able to solve it with signature campaigns (and im still worried that I should have been paying taxes because it would be considered a wage in exchange of a work and not an investment). They may decide to tax you with an extra for all the months that you didn't pay taxes on what is considered a wage when they realize you had a lot of payments (because once they have your bitcointalk account, they can trace all of your payments ever here, not just what you are reporting).
I can't imagine tax authorities asking you for your Bitcointalk login or private keys. They certainly shouldn't, and technically that wouldn't prove ownership of the funds at the time they were paid. I'd archive the sig campaign threads in case the site gets nuked, but that's more about proving where funds are derived from and less about taxes.
I think the IRS really just cares that people are making a good faith effort to pay their taxes. It seems like signature campaigns would fall under regular "wages/salaries/tips" or "other income" and taxed at the standard tax rate for the cost basis, and capital gains for any gains from holding.
They won't really care. None of them care. You just tell them you have a screenshots and few bits and bobs that'll let them offload the problem on someone else if it ever arises, and it won't. They won't even ask to see them.
Yes they will care and it's only going to get worse. Any money without a dubious origin is at risk of being confiscated so you better be able to prove you obtained it legally.