As I understand it, the responsibility of taxing the vehicle remains with the registered owner at the time of the offence, unless the previous owner completed a SORN I would suggest its their problem. You could simply deny the car was not actually in your possession for the very reason that it wasnt displaying a tax disc and having paid the readies the agreements witht he buyer was you collected it when they found the disc.
I am only basing this on a situation I had. I returned a taxed lease car to the leasing company before the tax ran out (1 week left etc), they didnt tax the vehicle as it stayed in their compound and even though I had receipts proving I was not in possession of the car and they collected the DVLA made me pay an £85 fine because I was still the registered keeper at the time the tax elapsed.