If enough people were selling scarce products such as some villagers and hybrids, then the prices wouldn't be so crazy to begin with. If flowers were offered for lower prices, they would immediately be scooped up and hoarded in some cases and then there will be nothing left at those lower prices and the higher priced stuff will still sell. If there is more supply of a product than there is demand for that product, then there is no need for a guild because sellers would have to undercut each other to sell their product. But for low supply, high demand products like Raymond, there is no need to undercut for the time being and selling him way below market value will just lead to a stampede of people trying to beat each other out for him.
Furthermore, if there are no strings attached to being in the guild, then what's the point? Even if some members will only sell to others in the guild, it doesn't matter if I can just join the guild to buy cheap products from guild members and then continue to sell at market value as I normally do. Why wouldn't I? It would basically be free money. I'm not trying to bash the idea; I'm just trying to be realistic about what will happen. At the end of the day, we'll still have the same situation where not everyone will get what they want when they want it.
This.
The reason I know why an open system wouldn't work is because I'm well versed in a game called Magic: The Gathering. Magic's market is close to stock market, but far more wild, as there's no regulations. Buyouts to inflate the prices are common, and people hoarding stacks of sealed boxes of specific sets also are very common.
There's a rarity system that by itself makes some cards harder to get than others, but then, there's an added layer of desirability called "does this card see play?", which means better cards (which more often than not, are also the least common cards) are always in high demand.
People try to get their cards from others who are new to the game or simply don't know how the pricing works, so they can flip those cards for more profits. Should a guild in this model be created, richer players and store owners would buy everything at once to flip at higher prices, because the more money you have, more chances you have to control the market and skew it in the direction you want it to go.
Now, I can see how a closed environment could work, but that would also need to be created and maintained based one trust and respect between the players. It's easier to set this up with friends that are closer of a small group of like-minded people, and realistically, even if said group could attract more people with the same goal, expanding too much would inevitably destroy the whole thing from inside out, as the more people you add to the group, there's more chances for one of them screwing everything up.
I don't know if I'm being helpful or not, but I just wanted to point this out because I can see that this thread was made by someone who really cares about the game and it's community, and I really like to see a change for the better, too, even if I think a change like this would demand more strict guidelines.