Honestly I don’t see why it need to be inside a bricks and mortar building?
people seem to want it to be near somewhere to ride after, you’ll not get much more local to the trails than my proposal, renting a village hall for the morning Vs a patch of land and a marquee even closer to the potential punters normal stomping ground seems like a no brainer…
If you could guarantee the weather I’d say just have it in the open air and save even more overhead costs (pun intended), better yet just make each seller responsible for keeping themselves dry, and provide their own table/tarp or whatever, more like a car boot than jumble sale…
How many tables/pitches are you really that likely to need 10, 15, 20 tops maybe? it doesn’t need to be massive.