Difficulty erasing hard drive/data on MacBook Pro (Feb. 2010) for trade-in

Hi! I have a MacBook Pro from February 2010 and have been offered money for trading it in. I backed up all my data and am not having luck erasing all my personal data, etc. from the laptop. I have tried to figure out how to do this by googling, but here's what happened:

When I restart, I try pressing command + R but this does nothing. I've gone into the disk utility through settings, selected my hard drive, selected 35 swipes, and hit erase, but it comes up with this error message, "System extension cannot be used. The system extension "/System/Library/Extensions/AppleUSBEthernetHost.kext" was installed improperly and cannot be used. Please try reinstalling it, or contact the product's vendor for an update."

I'm not tech savvy and have no idea what this means. Can someone generous help with easy/clear instructions, explanations, or tips?