Question: What are some good sites that list the value of old stamps ? I have an entire sheet of Apollo 13 + tons of others, some in a book, that belonged to a relative. Thanks.

Answer: You will get face value (at the most) for these sheets. There are plenty around from this era on ebay and most are all sold at less than face value, with a few exceptions. Look at ebay for a few of them to get an idea.
Or visit this link or this one