system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do online retailers such as Amazon and Ebay confirm the legitimacy of their luxury items?
gpt
In short they don't.  They're just a middle man.

Some of the sellers on those sites depend on their reputation.  The others can use escrow or just wait for someone who doesn't care.

Others, for very high end and rare items, can use escrow.  This is a third company with a good reputation.  When you buy the good the seller gives the item to the escrow company.  The buyer gives the escrow company his money.  The escrow company validates the authenticity, then sends the money to the seller and the good to the buyer.

The tests run are typically spelled out in the sale contract, as well as who pays for the escrow service and other contingencies.