I do not believe any of the auction houses slab them, they will likely rely on the seller to do that before it comes up to the block for sale. As @csaag mentioned you will have the option before slabbing the books to have them pressed, depending on the condition and age of the book it could definitely help you with selling them.
If you want to do more you can also take some good pictures of them and post them up on the "Hey Buddy, Spare a Grade?" forum. You will want to post 1 book per thread and then our members will look them over and give you their opinion of the condition.
Welcome to the forum!