How to avoid being taken advantage?

Some so-called potential buyers attempted to take advantage of sourcing agent service. They asked a sourcing agent to source the products they were looking for, but disappeared when they got suppliers information. Sometimes they might just want to do some market research before they started a new business, and intended to make an agent a fool to do this for them free of charge before they ran away. It feels disgusting when you meet someone suspected to behave like this. However, is anybody willing to share how you would do to prevent it from happening? How about requesting to prepay some service fee?