TRUST is the issue - they are scared you will nick their work and you are scared they will rip you off.
Get them to show you a list of fifty addresses - no names - no prices and say "I want to pick any five from this list and you take me where I choose and you introduce me."
If he is really selling genuine stuff he will be happy to do so and if he wants it kept quiet from the custy you can be introduced as an assistant or manager or something.
If introduced you can ask the seller in front of the custy "How often do you call here?" Smokes out being charged for two monthly stuff at monthly prices.

If that all works out then you can pay say 25% at the point he hands over the lists with names frequency and price - plus you agree to pay the balance on a monthly rate over the next three months while you work together and handover.