Hard to help with so little info

Why should the seller be out of pocket when it was your receptionist who caused the issue?

Why are’nt you asking them for the money when they are at fault here?

BUT also

How long did you give the seller to respond?

If you sold something and later got a message saying’ my receptionist accidentally returned this, I want it sent back’ 

what would your reaction be?  Especially if there was no offer to pay postage again

How did the receptionist manage to accidentally return it?

Was your name on the package.or just no name and the address?  Had they been told to return other mail and included this by mistake?  Did they take it and return it themselves or refuse to take it off the delivery person