Sentence:  Of all trades and arts in repute or possession, / Humbugging is held the most ancient profession. / Twixt nations, and parties, and state politicians, / Prim shopkeepers, jobbers, smooth lawyers, physicians, / Of worth and of wisdom the trial and test / Is—mark ye, my friends!—who shall <WSD>humbug</WSD> the best. 
Meaning of the word marked with <WSD> tags:  To play a trick on someone, to cheat, to swindle, to deceive.