Auction houses can publish their sales in these portals, additionally to their own websites. They may charge a fee to buyers bidding from these sites.
Auction FR
https://www.auction.fr
Auction Network
https://auctionnetwork.ca
Bid Spirit
https://bidspirit.com
Delcampe Prestige
https://www.delcampe.net
Philasearch
https://www.philasearch.com
Stamp Auction Network
https://stampauctionnetwork.com
Stamp Circuit
https://www.stampcircuit.com