I agree with Oldman in the statement that most people don't look at creds. If a person has a blackbelt (provided that art uses blackbelt to mean the upper ranks), most people will not question further.
Now, those that understand the varying abilities between instructors (notice I didn't say 'Style'), may want to know a little about association or even lineage. Lineage should speak for something. Anyone I promote to blackbelt will meet a standard that I expect. There shouldn't be a decline in quality.
Association doesn't mean a whole lot. It is sometimes a plus for prospective students to know they belong to something bigger.
As for credentials for myself, I usually tell people just a few things.
1) I am a student of a true Okinwan master - The only person in the Greater Dayton, Ohio area directly linked to Okinawa and the U.S. Director for his Okinawan Shogen-Ryu Karate-Do Association.
2) I am listed in the Okinawan prefecture registry of True Okinawan Karate instructors.
3) I hold Masters degrees in Anatomy and Physical Therapy (in 8 weeks). So they will learn safe and effective martial arts.
4) I let them watch/participate in a class and see what I can teach them.
If they don't want to be my student after that, Fine. I don't teach for a living.