In chapter 9, the Artful Dodger and Charlie Bates return from “work,” and Charlie has stolen some handkerchiefs, or “wipes,“ as he calls them. Fagin examines them and says that Oliver will have to learn how to remove the “marks” in the handkerchiefs with a needle. I assume what is meant is that the handkerchiefs are embroidered with the owner’s initials. Oliver’s job will be to carefully pick out the stitches, so that all traces of the previous owner are removed. In this way, the handkerchief can easily be resold.
It may seem odd that so much effort would be put into a stolen handkerchief, but in the first half of the nineteenth century such items were handmade and relatively costly.