Most online auction sites offer merchandise that is the property , much as an auctioneer would at a public auction.

The answer is "of others".

Explanation:

The auction site has become a service, in which auction clients or members are using the internet to buy or sell goods and services.  

These sites use the different auction sites, which include forums for consumers, operated by multiple software, that's sale forms also recognized as a virtual auction. These sites sell items owned by the others, which is an auctioneer does at a public auction.

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