You forgot the possibility of exchanging non crypto but real assets. What will be a bomb in this market.
What will the developers do if people use the service to sell illegal goods / services?
Difficult question. In different countries, different legislation and even a list of prohibited things / services is different. It's probably the user's responsibility in using the service with illegal purposes.
We do not create a service for this, but we can not control the application, probably someone will trade in prohibited goods.
The creator of the automatic rifle is responsible for the person who shoots it?
We develop technology and users use it.