Allocate the $20 among your old clothes that were in the bags, according to their apparent value. Return the money she paid for the items that she claimed were in "bad condition" and say you're sorry she couldn't use them.
A reasonable fraction of twenty bucks is not going to impoverish you. You might as well satisfy your buyer.
Unless your used clothes are in wearable condition, and made from good fabric such as wool, linen, silk, or 100% cotton, you probably shouldn't try to sell them sight unseen. Donate them to Goodwill or the Salvation Army instead, and ask for a receipt for a tax deduction.
Many people who reply are saying, "Bags of used clothing are generally sold 'as is' ". They're right about that, but you really shouldn't take advantage of a person who has to buy used clothes. Don't try to palm off your rags.