Our coop is a bit different - it's in a church, so it's only 'open' two hours a week. Everyone works some.
Our membership fee is $40/year, and our markup is only 5-10% above wholesale.
Work is often only 2 hrs/month - and it can be setup, running the cash register, clean up, ordering, etc.
We buy a lot of produce from a farm that is 2 hrs away - close enough to be 'local' but far enough that no one wants to drive there themselves

That farm also is a produce distributer so we can get bananas, avocados, etc as well as the local spinach, kale, berries, tomatoes.
We don't just do produce, but we try to limit the dry goods we have on hand due to lack of storage space. Most dry goods are special ordered. Special orders are only as good as you choose them to be - our dry good distributer sells lots of things - including highly processed boca burgers, etc. We buy Annie's whole wheat organic mac & cheese by the case, yum.
You'll have to go ask your coop how they do things - there's a lot of variety as to how coops work.
Aven