As far as the exchange is concerned, when you transfer the corn to cold storage that counts as a sell I believe. That is the point where the paper trail ends as far as I know. Maybe #[2] can comment
Discussion
Definitely NOT a sell, as there was no exchange for fiat. Are you sure the exchange listed it as a sell? If so, what exchange? I hardly ever use centralized exchanges, but I'll be sure to avoid any that does list withdrawals as a sell.
If you look at something like taxbit, transfers in and out are tracked differently from sells and buys. But tracking fees is the annoying part (to me).