One reason suppliers maintain reps on the road is the results they can achieve by being in front of retailers. Some people find it harder to say no to someone in person than by email or on the phone. Too often I see retail businesses, not just newsagencies, overloaded with stock that does not deliver the return necessary.
If a rep claims a product is working particularly well, ask for proof. It is your money. Satisfy yourself the claims are true and that you can expect such results for yourself.
It is okay to say no, even to a rep who has become a friend.