Unless the ad read "Cracked blade!" somewhere....and you just didn't read that it was a junker...

SEND IT BACK! NOT YOUR FAULT!

Honestly, as a seller from time to time myself I want my customer to be totally happy. They buy something from me and don't like it, I'm going to give them back their money or trade it for something they like better- even when that means doing very 'fairly' (read: I lose money) priced or pro bono repair/replacement/refitting.

I would be very sad if someone had a problem with my razor and didn't tell me.