I appreciate the concept and have executed the same thing but was dealing with millions of dollars among companies as opposed to individuals.
I think it would be a struggle at best, and a nightmare to administer. You would be hard pressed to get manufacturers interested, while you may think talking 500 propane tanks is an impressive number think of the number that CTC or Cosco buys..... if you are not going to inventory anything than how do I get my tank when I need one if the co-op made it's purchase 3 months ago and everyone bought enough to last them a year or more? No manufactureris going to be your warehouse, they have distribution channel to market for that.
Just my 1 1/2 cents