Hi yongwen,
Welcome to the Community!
Anytime someone accepts a payment there is a risk that the payment may be reversed due to a chargeback, buyer claim or bank reversal. We encourage sellers to establish safe selling practices and that they are familiar with PayPal's Seller Protection Policy.
Please visit our security center at https://goo.gl/HZVkpS. Be sure to select "I'm a seller" to ensure you see the Seller Protection Policy along with some safe selling tips.
I hope this helps!
-Leslie
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