It would, in theory, be a not bad idea at all. In practice, it would require working out the estimates of sales and how effectively the business would draw away existing business from the regional photo film retailers. You would probably want to make this a nation-wide scope, not just GTA-regional or Ontario-only. I do have experience with two different types of co-operatives (grocery and housing), and there are many kinds of co-operative business models. If you haven't yet, google "seven co-operative principles" and use that as an anchor to envision your business plan, if you so decide to pursue it.
For something like this, the model would probably benefit best from member-owners and non-members being able to buy from the co-op, but only member-owners are able to buy product at lower prices, receive exclusive deals, and if the co-op hosted darkroom courses, then pricing for member-owners would be nominal, and non-members would pay a fairly high rate comparable to other places. Net profit per annum, if there is any, can be paid out to all member-owners proportionate to how much they spent that year relative to all other member-owners.