Poor Susan. There really isn't much else to say about her. Her simplicity and meekness are what drive Henchard to auction her off (she's just too nice!), and her simplicity and meekness are what make her accept the auction as a binding agreement.
The narrator is very insistent about this last point: Susan isn't to blame for going off and living with Newson after Henchard auctions her off. Of course she doesn't have to leave with the sailor. Women might not have been treated well in late 19-century Britain, but they weren't slaves. You couldn't just sell your wife. Sheesh! But Susan thinks it's her moral duty to go with the man who paid for her. She leaves with Newson, "apparently without a thought that she was not strictly bound to go with the man who had paid for her" (1.80). The narrator repeats this several times: Susan didn't know any better. The narrator acknowledges that it's hard to believe:
It may seem strange to sophisticated minds that a sane young matron could believe in the validity of such a transfer. (4.2)
No, Susan isn't insane. And no, she isn't stupid. She's just very meek, very innocent, and very trusting. In his hungover anger the morning after the auction, Henchard blames Susan for what happened: "'Tis like Susan to show such idiotic simplicity. Meek – that meekness has done me more harm than the bitterest temper!" (2.6). If Susan had more spirit or a more "bitter temper," she might have stood up to him. She certainly wouldn't have just grabbed her baby and left the tent when Henchard offered to sell her. But once the money changes hands, she accepts it as a done deal. And once she leaves with Newson, she thinks that she's bound, both morally and legally, to be a faithful wife to him:
The history of Susan Henchard's adventures [. . .] can be told in two or three sentences. Absolutely helpless, she had been taken off to Canada, where they had lived several years without any great worldly success, though she worked as hard as any woman could to keep their cottage cheerful and well-provided. (4.3)
Even though she's not Newson's real wife, she feels and acts as though she is. Later, when Susan believes Newson is dead, she thinks she is a widow and therefore free to go back to Henchard.
You might have noticed that the narrator keeps insisting that Susan is not to blame. Why does this matter so much? After all, Henchard treated her like garbage. Why would it matter whether Susan really and honestly believed in the validity of the "transfer" of her marriage?
Well, it mattered big-time in terms of social and moral codes of the 19th century! Living with – and having sex with – another man when you were already married was adultery (think The Scarlet Letter). Even though Henchard treated Susan terribly, it would have been considered even more terrible for her to go off and sleep with another man that wasn't her husband. But because Susan was innocent and trusting enough to believe that her marriage was somehow transferred when Newson paid Henchard, it's clear that she isn't guilty of intentionally committing adultery. Phew! Thank goodness for our Victorian morals.