I am new in Magento environment and wanted to ask you folks, if you can help me with this problem. I have a client that doesn't allow me to access his magento sourcecode and alter it. The only way for them is to develop by me a plugin/module for Magento, to enable cart and checkout for mobile users as Rest API. I wanted to ask you is this possible and could you please point me out some tutorials for that task? Thanks in advance!
You can create a Custom API for magento as a plugin.
Look at this for reference.
Hope this will help you.