Buying, selling, trading, and commerce. That's what makes the world go round in this play, and not the so-called heroic deeds of men. In Troilus and Cressida, the Greeks and Trojans tend to treat people (especially women) as if they're marketplace items that can be bought, sold, stolen, or traded. The Trojan princes talk about Helen like she's an expensive piece of silk fabric listed on eBay; Cressida is treated like a commodity when she's traded to the Greeks; and Ulysses talks about Ajax like he's a product the Greeks are trying to sell. Ultimately, we think that Shakespeare wants us to consider how we determine a person's "worth" and "value." Do people have some kind of inherent or automatic "value"? Or, does a person's "value" depend on whether or not other people think they are "worth" something?
Achilles is afraid he's worthless because he believes that a person's value depends on his reputation, or what other people think of him.
Over the course of the play, Cressida is portrayed as a commodity that can be bought, sold, borrowed, or traded. Same goes for Helen.