It is better to contact the seller before the purchase and agree that you'll open the parcel and make the COD payment on certain guidelines.
Also, ensure both of you establish these guidelines clearly for refusal (Eg: reject it if it is different from advertised and on what basis, item is not new, probably not because buyer changed his/her mind, etc) that will safeguard interests of both the buyer and saler and ensure a smooth transaction.
As a seller, I also take pics of item packed and send it to buyer with shipping details which also helps in some cases. You can request the same to your seller too.
Bottomline, clear and honest communication upfront saves a lot of agony downstream.
Good luck!
