A reputation for actually being competent.
I think the best thing one can do is to show the prototypes and exhibit sample builds. It'll cost to set that up without immediate return, but I think the promotion (and actually demonstrating competence) would be worth it in the long run.Fair, but what about new businesses or up-and-comers? Here's a scenario ;
Luthier X wants to start his own guitar-making business/shop. He's trained at a reputable luthier school, Roberto-Venn for example, or apprenticed under an already-established luthier/shop and has worked for a major brand as Position X (guy that glues necks or handles fret dressing, etc.) while working as a local small music shop's on call tech for 2-5 years. Lutheir X gets his tools together and/or gets a loan and has a reasonable stock of woods and parts. He has a couple "stock" designs and wants to offer customs.
What are some things he can do to draw your attention and business, over Luthier Y?