Have been looking but I can't find it can someone help me

Any auctions on site are run through forums just like this! Whoever made the thread keeps track of all the bids and pings people if they were outbid and so on. Although auctions like that are not super common. Those can be found in "Pet Marketplace" and "Item Marketplace"

However for most items they are sold using the trade broker, where the items are sold for a set price.

The other auctions that you may have seen is the tab for "eggling auction" Eggling Auctions take place during festivals. They are mystery eggs (themed) that have set starting prices and set bidding increments. These auctions are announced and explained thoroughly in a post by Krinadon every time they happen.

I have linked the forums that I was talking about above, click on the blue phrases.

Hope this helps!