Have a look at the general appearance and cleanliness. Sometimes the casings will have cracks. What about the terminal conditions? Any telltale signs of abuse or excessive corrosion?
Has the seller got the cables that connected them together too? If so buy what you can as they will be professionally made and dependable.They can be costly depending on size and type of lug. Use only copper.
Maybe you can get a better deal if you buy quantity. I'd put one in all my vehicles so send me 5 for now, lol.