After my February 26th oopsie (see Mainland land holders will soon be able to auction their land), Linden Lab confirmed on March 26th, 2019, that user-to-user Mainland auctions are now live.
The auction system leverages Second Life Place Pages as the medium for presenting land for auction and for placing bids, together with a new “cover page” listing available parcels up for auction. which can be found at https://places.secondlife.com/auctions.
If you are not familiar with Place Pages, you can find out about them via my Place Pages tutorial. This will be updated to include the relevant information on setting a parcel for auction in the near future, once the service has been confirmed as being “live”. In the meantime, additional details on Mainland auctions can be found in the Second Life Knowledge base as follows:
Again, and for clarity, do please remember, the auction system is for Mainland only. Private regions or parcels cannot be offered through it.