The title Sold—at first glance—references the sale of Lakshmi. The poor girl changes hands time and again, commanding a higher price from each subsequent buyer until she lands at Happiness House where Mumtaz sells her young body over and over again. As we read the book, we understand that more than just her body is being sold, too—her innocence and access to education, amongst other things, have also been stripped away from her for a price.
The word sold operates in another way, though—in a way that, without reading the book, might come to mind. Have you ever been in an argument with someone and, after trying to convince them of your side, had them respond by saying sold? It's basically the same as saying you convinced me. In Sold, despite being enslaved, beaten, raped, and more, Lakshmi is never completely sold on the idea that she has nothing left to hope for—in other words, she's both sold and not sold.
And you thought it was just a simple one-word title.