Right now we are running some BFL, Dharma Trading Company, Northern Essence, Treliske, Superwash Merino, and Targhee. We also run Peace Fleece, Andee, and Licorice Twist - but these are closed at the moment.
We try and find places that give discounts for large quantities and buy yarn together. I.E. Usually I don't need 40 skeins of something, but one company gives a discount at this level, so we order 40 skeins together, everything gets shipped to one person and then shipped to you.
There is no prepaying - you pay for your yarn and shipment of the yarn when the coop closes. Sometimes there is a fee (that covers shipping supplies and gas to PO) but not always. This is always stated in the file about the specific coop.
If you are unsure, you can always join and just watch for a while to see how things are run. Our main goal is to get yarn at a cheaper price, but not have to go through order the large quantities that we would have to do as a seperate person.