I wonder how I am supposed to sell bitcoins on an exchange. I have been considering several and have gone through the KYC/AML process for three exchanges. The problem is once I sell my coins they might ask me to prove where I got them from. Keep in mind I am not intending to withdraw the FIAT, I just want to sell high and rebuy lower, and withdraw the coins.

The problem with proving the source of my coins is, how the hell do I do it? I bought them on MtGox and they can't ask MtGox for verification. Even if they did, the coins have gone through several addresses over the years. I can't even trace it back to the MtGox purchase since they went through other exchanges (and got mixed).

So what is one supposed to do here?