Fraud

I transferred ingredients for an enhancement from the Alliance to the Horde through the Booty Bay auction. Horns, like a rat, waited for the right moment and bought them. Is there anything that can be done about this?

Anything expensive put for cheap on NAH gets sniped by bots or players. Either keep the price near or at the original cost, or find another way to shift items between factions.

There's no support for this.

You posted something to the auction house and someone bought it. That's hardly fraud.

I recommend not using that auction house. It is often camped by people looking for people like you.