Who is actually selling it?
Large marketplaces can contain many third-party sellers. I look past the familiar website logo and check the seller and fulfillment information. If the brand publishes an authorized-retailer list, even better.
Korean products change
Packaging and formulas can be updated, and versions may differ between markets. I compare the complete name, size and current packaging rather than assuming an older review describes the bottle in front of me.
I keep the order information
When it arrives, I look at the seal, printing and date information before using it. I save the receipt and photograph anything that looks wrong. That makes a return or authenticity question much easier.