I am trying to put myself in the OP's position. Had I been looking for a bike, if this one came available would I suspect anything unusual? Was the seller a known dealer in bicycles? If not, how did they just happen to have this bike and want to sell? Is the seller a rider or was there any reason a non-rider would just happen to have this bike?
Yes, try to get your money back, but make sure this seller never does it again. Contact the authorities. Let them handle it. Do it now and not tomorrow. The longer you procrastinate, the more complicit you appear.