If the product delivered to you is in any way different from the product you intended to purchase, you need to state so before you mark the order as completed. As soon as you receive the items, contact the party you traded with and/or start a dispute.
As soon as you have completed the transaction, you can’t return the product. You have to check if everything is alright with the products as soon as you receive them and fix all the issues with the seller.
I personally haven’t experienced such issue. However, in my opinion, if there is something wrong with the delivered products, you have to discuss with all the parties involved in the order process and find an appropriate solution for it.