The answer to this question can be found in Chapter Fourteen of Book the Second. Having heard the testimony of one Roger Cly, who sought to incriminate Charles Darnay with his evidence, only to be proven to be a liar by Stryver, we are told that this same Roger Cly dies soon after the trial, and is buried to great public delight due to his position as a "spy" and the general dislike that the public have of such people. Note what the text tells us:
At length, a person better informed on the merits of the case, tumbled against him, and from this person he learned that the funeral was the funeral of one Roger Cly.
Of course, this fills Jerry with excitement, as due to his unofficial job as a Resurrection Man, he is able to find an opportunity to profit in such a situation, as by digging up the grave of a recently deceased individual, he can sell the body to a number of different people who are eager to perform medical research or find some purpose for a dead corpse.