Can't boot Imac, blinking folder, etc

Hi

Seen a lot of information out there, but doesn't really help, and the search function is bugged. I am selling a Mid.2010 Imac and must delete my data and install the original OS (Snow Leopard) because Yosemite and all the thing is linked to my Apple ID.

I did it the usual way, restart, DVD, C, Disk Utility, erase, Install. It stopped in the middle of the installation.

Now, a restart gives a blinking folder. Holding C doesn't do anything. CMD+R neither. Holding Alt gives me only to alternatives, using wifi or booting from DVD. Doesn't work whatever option I choose.

I need to fix this today. It starts to be a bit too much with this **** Imac. 2 screens changed, 1 HDD recall, then it worked perfectly, but can't even be reverse to original OS. Unacceptable.