What kind of agreements are there between you as the retailer and the rep as the wholesaler? The company that the Rep sells for.....is it a captivenrep or independent rep ? At a minimum the company using the rep should be notified. The rest of the issue all depends on whether there are agreements in place that define the respective roles of the players and if there is restrictive covenants and no poaching type clauses.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained. Please click "helpful" or "best answer" if my answer added any value or add a "comment" if you have more info for me to help you get a better answer.
Since you buy through reps of other companies the only claim you can make is interference with prospective contractual relations. Yeah. It doesn't sound like the greatest claim but if you have been doing business with this rep for a while and he knew this was an important account (he obviously thought he would make more money with the other dealer) you do stand a chance. Remember Texaco?
UNTIL AND UNLESS A RETENTION AGREEMENT IS EXECUTED AND DELIVERED BY BOTH PARTIES TO THIS EMAIL, NO ATTORNEY-CLIENT RELATIONSHIP IS ESTABLISHED BETWEEN ME AND THE PERSON OR PERSONS POSING THESE QUESTIONS.