Forget ebay. People/shops sell super low-end Chinese and Kenyan junk there for $6-8/dz (shops PAY around $11-15/dz for most standard patterns if tied in one of the better offshore locations like Thailand), and there's no way to compete with that. The only market for USA-tied flies is if you do something new/better/different and sell either wholesale to a shop or direct online.

And the excise tax is on the first sale of any fishing tackle, period. It could be thread on a hook that you already caught 50 fish on and you'd still need to pay the 10%.