Answer:  Online auctions avoid the time and expense of holding an event (no venue, caterer, auctioneer, AV equipment, event night volunteers etc) so most of the "savings" come from activities that are handled outside of the software.    The software handles the "auction" aspects of your event - and gala and online auctions are more similar than they are different.  

Both need:

• Donors

• Items 

• Mechanisms for:

  • bidding and 

  • awarding items to winners 

  • processing payments 

  • issuing receipts 

  • reporting results   

And the differences generally replace one task for another:

The majority of tools and settings within the software apply to both gala and online events.  If you are running a stand alone Online Auction, you can winnow out tasks and instructions that apply only to Gala events by referring to the Online tab on the Help Overview Map on your site's Dashboard 

Additional Settings for Online Auctions:

• Customizing Your Catalog Display
• Customizing Bidding Options
• Setting Up Online Payments
• Setting Up Access to Bid
• Promoting Your Online/Mobile Event
• Sending Invitations to Bid