Why is it a fake? A couple of thoughts from a non-expert...
The horn handle is in remarkably good condition considering how badly the blade is pitted.
The blade looks artificially aged.
The long shallow clip grind and the wide blade with the narrow clip look more "Iron Mistress" than real Bowie.
What do the bidders plan on doing with it? That's easy... sell it a a genuine Bowie on Ebay.