In this situation, it seems like you have done your due diligence as a seller by testing the air compressor and showing it in working condition before the buyer purchased it. Additionally, you mentioned that you clearly stated it was sold "as is" on Facebook Marketplace.

When selling used items, it is generally understood that there is a certain level of risk involved, and buyers should be aware of this. By stating "as is," you are essentially informing the buyer that you are not responsible for any issues that may arise after the sale.

However, it is important to consider the buyer's perspective as well. They may genuinely believe that the issue with the motor occurred shortly after the purchase and may not have been aware of any potential problems beforehand. It would be helpful to have a conversation with the buyer to understand their concerns and assess the situation.

Here are a few options you could consider:

1. Offer a partial refund: If you believe that the issue genuinely occurred shortly after the sale and want to maintain a positive relationship with the buyer, you could consider offering a partial refund to help cover the cost of repairs. This gesture could be seen as a goodwill gesture and may help resolve the issue amicably.

2. Suggest troubleshooting steps: If you are confident that the air compressor was in good working condition when sold, you could offer some troubleshooting steps to the buyer. This could include checking electrical connections, fuses, or providing guidance on potential motor issues. By offering assistance, you show that you are willing to help, even though you are not obligated to do so.

3. Stick to your original agreement: Since you sold the item "as is" and made sure it was working before the sale, you have no obligation to offer any refund or assistance. You can politely explain to the buyer that you tested it thoroughly and cannot take responsibility for any issues that may have occurred after the sale.

It's important to approach the situation with empathy and understanding. While you are not obligated to offer any resolution, being open to communication and finding a mutually agreeable solution can help maintain a positive reputation as a seller.