Book stores often maintain used books sections. You can sell these books to the stores. They will offer you a pretty low price but you will be able to sell them none the less. If you go to a bookstore of a college, they are going to be more willing to purchase what you have on offer and will be offering you an even more reduced price than a regular bookstore.
Auctions sites are an excellent place to sell these books. Sites have millions of users and someone is almost always in need of a product that you have. So you will have a better chance of selling these books on an online auction site. You can find a reasonable price as well, definitely no worse than what the book store will offer you.
You can also sell these books directly to the students if these are related to course work. They will be more than willing to pay you a decent price as the same used books in a college bookstore will cost them a ridiculously high price. You can go to classes to sell or just paste the books list and your cell phone number on college announcement boards and wait for the phone to start ringing. Timing is important and make sure you do this at the very start of the semester when most students are looking for books. If the first week of classes is gone, your chances will be very slim of selling them.
If the books are really old and no one is willing to buy them or needs them, you can sell them to a recycling plant. Since the law books are really big, you should get some decent money for them though often the rates offered are ridiculously low. Anyhow, you will still pursue something.