Trading can work in several ways. Trades are done from user to user. The currency of trades can be both diamonds and/or gold, depending on what the user asked for in their thread. Although pet trades with diamonds are rare, it is not impossible, and it is allowed. Though rest assured that the majority of trades are done in gold

When bidding for a pet, make sure that you will have the money on-hand or when the auctions ends to ensure that the person who owned the pet will not be scammed by false bidding. If you won a pet auction, the user will notify you by pinging you, and setting up the pet to your user ID so that only you are allowed to buy the pet at a certain price.

Setting up pets
This is the method of setting up a pet for a user. First, go to the pet's profile page and click on 'Manage pet'. A window will pop up displaying options such as 'Sell price', 'Breeding price', 'Sell to user ID', etc.

Input how much the pet will be in the space saying 'Sell price'. Afterwards, input the user's ID into the space called 'Sell to user ID'. To find the user's ID, simply go to their profiles and you can find it in the User description.

If you are buying a pet from a user, it's a lot more simpler. Just go to the pet's page and click on 'View price'. If the pet's set up for only you, you will find that other people cannot buy the pet. If the pet is free for all, then it will be sold in a first come first serve basis. Just click on 'Purchase', and you will receive a notice in your 'Account' tab that you have purchased a pet from such and such user. The gold will be deducted from your account immediately after purchase, and transferred to the seller.

And wow. That was a bit long winded Hope it helped anyway :3