I found another 2004 that won't start so it's selling cheap.
 

I recently got in a wreck with my 2004 infiniti g35, while searching for another car I found another one that's all in one piece. The description says "NO TITLE PARTS ONLY Selling for parts 2004 Infiniti g35 it’s in good shape the security system is set off so it’s not letting it start" when asking the selling he tells me that he was told the key needs to be reprogrammed. My g35 has all the parts except the front passenger side tire assembly and headlight. So my question is, what is causing his not to start? And what can i do?

Troubleshoot the problem then fix it.

See, I'm thinking couldn't I switch the ignition from mine into the new one?

what’s the point of swapping parts without knowing and diagnosing the problem? You can end up just chasing your tail for weeks.