Where can I find the value of my old German postcards?

I have more than a dozen Arthur Thiele postcards. Many of them have animals dressed like humans, doing human things. I know these were printed around the turn of the 20th century, it looks like during the 1920's from the clothing and technology shown. Some are cat postcards, others are dog postcards. I've seen the prices on Ebay, but I'm wondering how close those prices are to realilty. Is there a book or website I can go to to check the values?