This could be a daft idea, but I can't think of a better situation than flytying for it to be a great idea!
Anybody that ties flies, ends up with a huge amount of surplus materials that they inevitably hoard just in case a fly in the future presents itself from another tyer, that proves to be deadly. I think dubbing in its multiple guises is the most stashed and least used, over indulgence. Correct me if I'm wrong
Now imagine an arena where you could take all the items you probably never even realised you had! Then swap or sell them, to other obsessive, compulsive, like minded individuals. You'd have to get there early so you'd get round the whole place and see everything.
Say a £5 entry with a sectioned area of trade stands selling new materials for you to hoard...drab furs & feathers, synthetics in the latest neon, blinding fluorescent colours and all at trade prices!
Perhaps another section with some fly tying aces....woul this be a hit?