Your list.php file does not load any of the WordPress Core, at least there is no indication that it does based on the code you posted. Thus, WordPress classes and functions are not going to be available, hence the error.
Merely having a file in the theme folder does not mean that WordPress will load the file, or that when accessed the file will be loaded in a WordPress context.
Do the right thing and use the AJAX API for AJAX requests. That is what it was meant for. You are doing this the hard way, and the painful way. There are plenty of examples in the Codex for using the AJAX API and plenty of questions here about it as well. Get started. If you have trouble, edit your question with the specifics.
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