A Dealer a/k/a Retailer (e.g., Automated Services, Great American Pinball, Tilt Amusements) typically sells to Home Users and Collectors...
http://www.sternpinball.com/find-a-retailer
A Commercial Distributor (e.g., Betson, Shaffer) typically sell to Operators...
http://www.sternpinball.com/find-a-commercial-distributor
Note: Some Dealers are also Commercial Distributors (e.g., Game Exchange of Colorado).