Bookscouter will give you Amazon.com's trade in value, where you mail Amazon the book and get a gift card, they will not give you the used selling price because it's hard to compare apples to oranges. You may be able to get more for your book by selling it as a used book on Amazon, so be sure to double check that price. As an example, the trade in value of a book I looked up was $7.87 but used copies were on sale for $19.87.
Holly told me she started writing content in 2011. At the time, she still worked a full-time job but created content online part-time to supplement her income. Over time, she was able to double and triple her rates until she could quit her full-time job to write. These days, she makes bank as a freelance writer and teaches others to do the same via her online course, Earn More Writing.