In the Cyclops chapter of James Joyce's Ulysses, the narrator explains to one Joe Hynes that he is now working as a debt collector, albeit without great success:

"An old plumber named [Michael E.] Geraghty.  I’m hanging on to his taw [whip]⁽ⁱ⁾ now for the past fortnight and I can’t get a penny out of him.

—That the lay [occupation] you’re on now? says Joe.

—Ay, says I.  How are the mighty fallen!⁽ⁱⁱ⁾ Collector of bad and doubtful debts.  But that’s the most notorious bloody robber you’d meet in a day’s walk and the face on him all pockmarks would hold a shower of rain.  Tell him, says he, I dare him, says he, and I doubledare him to send you round here again or if he does, says he, I’ll have him summonsed up before the court, so I will, for trading without a licence [sic]."⁽ⁱⁱⁱ⁾

This debtor's defense reminds me of the following provision of the California Real Estate Law denying unlicensed real estate brokers and salespeople recourse to the courts:

"No person engaged in the business or acting in the capacity of a real estate broker or a real estate salesperson within this state shall bring or maintain any action in the courts of this state for the collection of compensation for the performance of any of the acts mentioned in this article without alleging and proving that he or she was a duly licensed real estate broker or real estate salesperson at the time the alleged cause of action arose."

Cal. Bus. & Prof. Code § 10136.  Similar California statutes apply to other licensed professions.  See, e.g., Cal. Bus. & Prof. Code § 7031 (contractors), § 7592.5 (alarm companies), and 10508 (mineral, oil and gas brokers).  The same bar applies to persons engaged in any business or profession for which a license is required under the Business & Professions Code governing the Department of Consumer Affairs or any board, bureau, commission, committee, or program within that Department.  Cal. Bus. & Prof. Code § 143.

(i)  The narrator is relating that he has held Geraghty's whip hand so that Geraghty cannot strike back. 
(ii) See II Samuel 1:19 ("The beauty of Israel is slain upon thy high places: how are the mighty fallen!").
(iii) The Dublin Corporation required that all merchants annually obtain a license.