Does anyone know of any good websites where I can find out how much certain old books are worth?

Of course. :)

Your Old Books is a giant research paper (with a table of contents) on the evaluating old books.

I also recommend Faded Giant, as @stranger_in_a_strange_land mentioned.

On the web fadedgiant.net is a good place to start. I’ve recently had to use a private appraiser to valuate my library for insurance purposes and I know she used this as a first-reference except for the rarest and oldest volumes. I can’t give you names of specific appraisers that would be of any use to you in Australia.

Wonderbook.com
They’re one of the largest seller of old books, and in my hometown. :)

BookFinder is the only thing I’ve been able to find so far. I’ll get back to you.

I agree, FadedGiant may be able to help.

@Spinel Thanks for that, I’ll look into it :)

@stranger_in_a_strange_land Thanks for the ideas. I might actually look for a private appraiser if I can’t find the answers on the net.

Use the Amazon search and check the offers of the used book section.

Bookfinder or ABEbooks.com. I have a fascination with the phrase “Faded Giant”, BTW, and have wanted to us it for something. A military codeword for critical nuclear reactor failure, ISTR.

abebooks.com is great. I may be interested in your military books. Would you kindly send a private message my way?