Probably something like the smaller producers sold locally, governing their prices in part by what bigger producers set. Meanwhile, large producers could likely pay for the price discovery, individually or in a group, or they had many contacts which allowed them to have an idea of what would be fair. But it was likely the US was stealing quite a lot as middlemen, which can now be at least atenuated somewhat by having many people trading directly and perhaps even more "state help" to figure out good prices long term.

I would be interested in understanding this better as well, and nice to see that the new system is already being used. Whoever is not using the new system yet will likely do the math to see how good it will be for them.

Edit to add another point: Large producers would likely buy smaller local producers' production as well in case the export price was good, so the smaller producers would increase their prices when they knew the bigger producers were buying a lot.