This is like the question of how to have a "bikini body", which is answered by having a body, and wearing a bikini. Take the vessel you made, add dirt and seeds and you have a planter. Drill a few holes in the bottom if you really want to.
If you have product ready to go that othewise would sit in your house/studio doing nothing, and you can afford to gamble $250.00, go for it! Write and attach your own price tags, or get written confirmation of the prices before the start of sales. Keep your own inventory list, and check any unsold product against it and sales receipts. Agree on a policy concerning breakage-who pays, how much, etc.
Write it up as a contract, get it signed and see if you can sell your wares.
If the seller won't give you a signed contract, walk away.
If you enjoy the company and interaction with other artists you might enjoy being a partner. The community aspects of a co-op are as important as the cost/benefit ratio. There could be ways to maximize the usefulness of being at the store-advertise the the artist on duty and have some work in progress that folks can ask about . Pump up your sales by show adding your work on those days.
And if you get your buy-in payment back, you could try it for six months and see how it works for you.
In your situation, I would find private studio space. Unless you want to try to patent your process, you can't really stop someone else from trying to replicate it.
That said, whatever work another person does with your process will stil be different from your work.
Imagine if the inventor of the potters wheel had said "I spent years making this thing work! No-one else can make pottery using this technique!". Or if there was a copyright fight over calculus.
In papermaking, there are different additives you put in your pulp to help it bind together. You use different ones depending on the type of fiber you're using. If you mix them together, you get some neat effects. I did this by mistake. Later I learned that someone tried to patent mixing those two additives together, as a unique process.
Personally, I like to share stuff. I share recipes and ideas all the time. Maybe someone else will do niftier things with my idea than I could.
At least a piece by Ai Wei Wei is replaceable, somewhat. Unlike the piece Ai dropped as a statement. Destroying the irreplaceable for political commentary is no more commendable than destroying current work is.
When does it crack? What shape is the object you are casting? Long, narrow stems will crack where they attach to the bowl of a goblet. Plates and bowls tend to crack across the bottom.
A few possible reasons:
The differences in thickness of different parts of the object are great enough that they dry at very different rates.
You are dumping the extra slip before the casting is thick enough.
You are letting your casting get too dry in the mold. You should pull it out when it is leather hard.
Do you use a defloculant? If your slip is sludgy instead of smooth and creamy the differences in viscosity can cause problems.