It all depends on the receipt, does it prove payment of the correct postage? Was there any postage on packet or had it fallen off?
If seller paid correct postage, then it really should be up to buyer to refund you and then claim off Royal Mail, as they have all the information.
Does the receipt show the correct postage for weight and value of package? If so using the copy you can try claiming on RM.
What you should have done is NOT PAY, then it would have become the sellers problem, instead of passing the buck to you.
Remember you can leave feedback and mark star ratings accordingly.
I must admit I always do my own postage and put selotape over the stamp.