The second part of the scene begins after Dogberry and Verges exit (after line 83). The watchmen see Borachio and Conrad (Don John's two companions) come in. Borachio and Conrad (who are both probably a little drunk), though, do not see the watchmen. The watchmen are thus able to eavesdrop on the conversation between the two villains. Borachio brags to Conrad how he has earned a great sum of money from Don John by pretending to have an affair with Hero in Hero's room while Don Pedro and Claudio stood outside the window. He tells Conrad -- and thus the entire audience -- that Claudio and Pedro believed that Margaret was Hero. Thus, Don John's scheme was successful.
However, the scheme goes wrong for Borachio and Conrad at that point, for the watchmen arrest them. Borachio, who had devised the clever scheme to fool Claudio and the Prince, also makes a clever pun at his arrest:
We are likely to prove a goodly commodity, being taken up of these men's bills. (156-57)
The word commodity has two meanings: it can refer to (1) goods that are being traded or (2) something that is useful. The word bills actually has three possible meanings: it can refer to (1) the halberds or weapons of the guards, (2) the warrants or documents of arrest, and (3) the documents of purchase for traded goods (a bill of sale). Borachio is suggesting that his capture will prove useful to the legal authorities of the town, but as prisoners they become not much more valuable than traded goods. Ironically, the clever and effective Borachio has been captured by the watchmen of not-so-clever and ineffective Dogberry.