It's frustrating but definitely not the end of the world. Ignoring it isn't an option, so you have to react somehow. The next question is: how exactly? It depends on the type of feedback you receive — there will be negative feedback that you accept and agree with, and feedback that you think is unjustified or just plain wrong.

People are different, so you should be ready for both cases. Now let's see how to handle each of them.

Justified negative feedback

Unfortunately, no one is immune to mistakes. For instance, you could reply after long delays, the product might be out of stock but still shown as available, or a customer might receive a damaged or wrong item.

In such cases, the situation needs to be resolved quickly and appropriately.

Remember that you should be professional at all times. Don't underestimate the power of an apology. You can't negotiate a reasonable settlement if you don't say that you're sorry and acknowledge how frustrating the situation feels for the client.
If you can't offer an exchange or a full refund so that the client edits the negative feedback, simply explain what happened. If potential clients see that you're ready to admit your mistakes and offer solutions, they'll see you in a more favourable light.
Take this feedback as a chance to improve something in your processes. Check your inbox more often, ensure that all listed products are in stock, and double-check the items you deliver so that customers' experience with you is only positive.

Unjustified negative feedback

At some point you may receive negative feedback that you disagree with. For example, the person can complain about something you're not responsible for, or about the product itself even though you described all the details in the ad and offered pay-on-delivery.

Such reviews are usually very emotional, so make sure your response improves your reputation rather than damages it. Remember that potential customers are not aware of all the details — all they can see is the client's dissatisfaction and your reaction to it.

It's crucial to respond calmly and professionally. Apologize for the inconvenience and explain the situation from your side. For example, you can show that you provided accurate details about the item in the ad and offered pay-on-delivery — so if the client inspected the item and paid only after that, you didn't trick them into paying in advance for the wrong product. Potential buyers need to know these essential nuances so they don't draw the wrong conclusions about you.
Try to reach out to the client and ask them to edit or remove the feedback. Explain the situation and offer alternative products if the person made the wrong choice. If you know you have other items that would fit this client, it's worth telling them about those too. You could also tell them what to look for when choosing similar products to ensure they meet their needs. Ultimately, we all want our problems solved. If you have the solution, you have a chance at a positive review.
Regardless of the situation, it's best not to be rude or dismissive. If you find that the customer is being abusive or threatening, you can request that the feedback be removed. To learn more about feedback appeal, go here.