As a full-stack developer myself with an emphasis in backend development and the owner of a small web development studio, I wouldn't hire you for back-end work. You're not a back-end developer, yet. As a back-end developer, you're responsible for 3 main components: Database, Server Configuration/Architecture, and Application Design and Development.
In the following paragraphs, let me try to give you an idea of some of the things you'll find in backend development. When working wth the database, you're not just writing SQL queries, but making sure that your database structure is efficient, optimized, and, if it's a relational database, normalized. You're not just setting up Apache and mod_php on a Linux box (or deploying a pre-configured machine), but you're helping to decide what servers will be the best for the project. You'll probably be responsible for making sure that any teammates have a consistent development environment and tools like vagrant, grunt, gulp, bower, composer, etc. shouldn't scare you. You may be working with platforms-as-a-service like Heroku and debugging against their idiosyncrasies or working with a unique cloud platform like AWS. You have to make sure that the system is up to date, and that there aren't any glaring security holes.
You won't just be writing SQL queries in your application layer, you'll have to understand which language and framework will be best for the job. You'll probably also work with an ORM or at the least a DBAL in your application. You should understand test driven development, agile methodologies, DRY, SOLID, and have incorporated those principles into your coding. You have to know how to avoid time-complexity and memory-complexity issues, and you must know how to mitigate the top 10 web application security risks (XSS, CSRF, SQL injection, etc.).
If you'd like to become a back-end developer, here are some recommendations:
Work with an established development team. I learned a lot when I worked with a couple of excellent development shops as a supplement to their in-house teams. They had some excellent developers who pushed me to learn more.
Don't start advertising as a backend developer if you've never had a backend job. Ease into it. Start by advertising full-stack development with an emphasis in front-end development. This is the position I'd start you in if I were to train you to do full-stack development (which appears to be where you want to go anyways).
Keep asking questions, but make Google your best friend. Also, you should spend a bit of time working on your marketing and sales process if you want to take on the complete development of a websites for well-paying clients. I've been freelancing for about 2.5 years now. I just recently figured out how to break the time barrier, and it's a learning process to do so effectively.