That serial number looks to be scratched out on purpose, probably because the frame is stolen.
It's almost laughable at how easily it is to tell that someone purposely scratched out that serial number. It would be one thing if there were a couple scratches on it due to wear and tear but that's blatant and at this point your main concern would be to help get that bike frame back to its proper owner by doing whatever you can to either contact them if possible or talk to your local police department and let them take it from here.
Whatever you do, DO NOT BUY THAT BIKE FRAME!
Just to reiterate, that frame is stolen from someone just like me or you that worked hard for our money and saw an opportunity to make some easy money for themselves.
If you have the person's name and phone number that is selling this bike it should also be forwarded to your local police station along with your suspicions that this bike is stolen.
Again, please do not purchase that bike frame.