Long time lurker, first time poster...
Recently I was in a "discussion" with a buddy of mine who is bible purist (believes it word for word). I told him I couldn't put any faith in a book that tells me it's an abomination to eat shellfish, but that slavery is ok. His response was that it couldn't be taken literally and that the times change. Who are we to decide what rules should still be followed?
There are a lot of problems with both the question you ask and his answer to it.
The question you asked (in the form of a statement) was "how could I believe in a book that tells me it is an abomination to eat shellfish, but that slavery is OK?" The problem with that question is that you're assuming that your preconceived notions of morality are correct and that the book (if it is true) should conform itself or portray itself similar to your own moral values.
Now, I would not posit that you should set aside your moral values for the sake of an objective examination of the book because that would mean having to admit your moral values may not be correct. But, if you truly wished to determine the truth value of the book, it might help to study the book in depth to see where it is coming from.
Second, his answer presupposes that your evaluation of the morality of the Bible is correct and therefore he comes up with the illogical and contradictory answer that "it couldn't be taken literally and that the times change."
So, in response to your claim about shellfish and slavery I would suggest two things. One, it is incorrect to automatically assume that a system which places shellfish consumption on a lower moral level than slavery is something you could not believe in. Two, the slavery that is allowed in the Bible is most likely not the kind of slavery that you are thinking of. Biblical slavery is very much like servant-hood. In fact, the Hebrew words for slavery and servant-hood, slave and servant, are the same.
Also, keep in mind that while a person is allowed to buy slaves, they are not allowed to break any of the Torah laws when it comes to the slave. Therefore, they must treat the slave like they would any other human being because the Torah does not allow an exception in their treatment just because they have been contracted for service.