First of all contact the seller in chat and try to solve the problem directly. Be polite and attentive to his arguments. If the seller still can deliver the order but after previously set date let him do it.
In case you do not have time to wait and you want to stop the communication with this seller you can contact arbitrage to acquire a refund and/or cancel the order.
A customer support agent will make a decision after reviewing your request:
full refund on your balance account in case of claims validity.
partial refund - depending on the quality and the amount of the work done by sellers. For example, if the seller failed to deliver the order in time but still completed the order.