Try making it really rewarding, like this maybe:
10 participants minimum with at least 6MH/s each.
6MH/s is about 40$~ in 18hours which means that at the bare minimum you'll have 60MH/s mining for you in 18h that equals to 400$ (worth of the card).
So with minimum participants and MH/s you'll earn the cards value (Still a win), any added participants and MH/s will just be a bonus.
That way the miners get a chance to win a GPU worth 400$ by risking 40$ only and you get to, at the very least, sell your GPU for it's market price if not make more.
Win-win situation.
Yah this sounds reasonable but I would make it 8mh/s minimum for 20hrs. I'm adding in a riser and free shipping that's why