Having a background in bricks and mortar retail and catalogue at one time long ago, have you thought how this Co-op will work, will it be strictly online? Will it have a storefront? If you are going strictly online have you thought out how you will store all the film, paper and chemicals, taken into account supply chain (pick and pack as well as shipping). At some point very quick it will outgrow your house and the Co-op could need a third party warehouse/pick and pack facility. All this will be costs that will have to be factored in.
In addition to accozzaglia's suggestion in talking to MEC on Co-op set up. I would also talk to Fotoimpex in Germany and other specialist retailers who sell film, paper and chemicals online.