In order to auction something, you have to have VIP.
If you want to auction an item, do this:
You'll then see the countdown start. Other models can start placing bids on the item straight away. If there's only one bidder when the auction time runs out, you'll get the price you first set up. If there are more bidders, the price grows 10% by each bid.
If no one has bid on your item and the auction is still running, you can edit the price or cancel the auction. Click "Edit auction" in your wardrobe or My auctions page. When someone has already bid on the item, you can't edit or cancel the auction.
You'll see an overview of your auctions on My auctions page.
When the auction ends, you'll get a goMessage. If someone bid on your item, you'll also get the goMoney or Diamonds you earned from the item.
If the auction time runs out and no one bid on your item, you'll get a goMessage. You can then auction the item again.
To make sure your auction is being noticed, write a post on the Auctions forum. You can also use the "Message to all" function. Remember that spamming others is against the goSupermodel rules!