To Shakespeare and his characters, wit was a show of intelligence. In numerous instances throughout his works, Shakespeare places a high emphasis on the ability of his leading characters to be witty, capable of hurling insults and jest with rapidity. A modern definition of wit means to know, or knowing. The inference, therefore, is that a wit's pedler is someone who deals in knowledge.
A pedler, or peddler, is defined as someone who offers either physical merchandise or intangibles, such as influence, for sale or trade. In the Elizabethan era, peddlers were not considered honest merchants. Because they did not belong to a guild or have a permanent place of business, peddlers were legally classified as vagrants. They were held in very low regard by the upper classes of society, particularly by wholesale merchants.
The character of Boyet is a lord in attendance on the princess of France and serves as both a scout and a sort of sparring partner. Held in high regard by the ladies of the play, he is very witty and helps them sharpen their own wits. He also serves as a scout and lookout for the princess and her party, warning the women of the imminent arrival of their suitors in disguise and of the plans that he had earlier overheard. A character with little depth, he is still an excellent vehicle for furthering the plot of the play.
The character Berowne, who utters the insult "wit's peddler" about Boyet, is actually much like Boyet. As Boyet is the adviser and helper to the princess of France, so Berowne is the right-hand man of the king of Navarre, her suitor. He speaks ill of Boyet only because he sees that he and his king appear to be blocked by Boyet from speaking with the women they are courting. As a lord, being compared to a classless peddler, a vagrant, would be a terrible insult. The stanza also suggests that Boyet's wit is not his own -- that he picks it up from here and there to use later on. The combination culminates in "wit's pedler."