If Correos will not accept the package it may exceed the allowed size or you may have not selected the correct weight in the ad.
The maximum weight allowed is 30 kg per shipment including packaging. As for size, if packaged in a box, the sum of the length, height and width cannot exceed 210 cm. Also, the length cannot exceed 120cm. Remember also that the minimum measurements for the package are 15x10 cm. The seller has to select the correct weight, taking into account the packaging.
If the weight is incorrect, the seller will have to cancel the transaction from the button that can be found in the Shipping section. Always do this before delivering it to the transport agency.
Once done, he will be able to edit the product and modify the weight information selecting the correct weight range.
In the case of a shipment cancellation there is no penalty for yourself or the buyer. Once it is cancelled the money will be automatically refunded to the payment method used by the buyer. The period in which the money will be available is between 1-5 days dependning on the payment method.
In case you need more information you can check the following articles: Size and Weight and Cancel a shipment once the offer is accepted.