Members say what they can help with, what they will do for free, and what they charge for. Anyone who needs something finds someone in their own church.
A church cannot host an unsupervised marketplace between its own members. Listings are read by someone at the church before anybody sees them, and every decision has a person behind it.
Prices are shown so nobody has to ask. What is agreed, and what changes hands, stays between the two people who agreed it.