Bought old Imac with Snow Leopard. How do I upgrade without buying new OS X?

I am brand new to Macs and I recently purchase a mid 2010 iMac that is in excellent condition. The problem is that I want to upgrade the OS from Snow Leopard but it is registered to the guy I bought it from. Is there a way (legally) to transfer the license to my Apple ID so that I can upgrade to Yosemite?

Welcome to Mac-Forums..

Ensure that you are running the latest update to Snow Leopard which is 10.6.8. You know you are running this update if you have the Mac App Store in your Dock.

If so, go to the Mac App Store and login as yourself and you should see Yosemite prominently displayed for you to download free of charge.

Once the installer is downloaded, it will try to run to upgrade your machine, you want to break out of it and copy the installer to a USB flash drive using these instructions.

Now, use the USB flash drive to get into the Yosemite installer, erase the hard drive completely and do a fresh install..

What exact messages are you seeing? Are you trying to upgrade the OS via the Mac App Store…and are you getting a message about the Apple ID you are entering (your Apple ID)..not being the Apple ID this particular computer is registered with?

This is what the seller of the computer is supposed to do before selling it:

Deauthorize your computer using iTunes - Apple Support

- Nick

Thanks, that solved the issue. Like I said, I am new to Mac so I don't know how everything works yet.

Glad that its sorted and we all have to start somewhere..

And welcome to the world of Macintosh and OS X..