People can sell their used car batteries to auto shops or car parts stores in their area that are willing to buy them. It's best to call around to those places to find out who does and what they offer.
Used car batteries are not worth much, but people can earn a little money or get store credit by selling them. Auto shops and car parts stores buy used batteries because they can refurbish the battery and sell it again. However, those places won't buy a battery if it is damaged. In that case, an individual's only recourse is to recycle the battery. Metal recyclers may be willing to offer a few dollars for the battery, but usually not. City or state requirements may have designated drop off locations for recycling batteries that are not sold.