1. Where do I see a list of all my friends?
2. Where do i buy items like health potions and nets to capture pets?
3. Can I breed my pets with my sister's pets?

I figured out how to buy items.

Your friends list is the contacts list in you inbox area

You can only breed other players pets if they are set up for breeding ( they will have a little pink heart when veiwed in the stables) if they are set up for breeding then you click veiw prices, click the drop down bar and select your pet you are using for the breeding then click on the breed button

New question:

How do I auction off items using the trade broker?

You cant auction them off on the broker, only sell for gold. You have to make a thread and keep track of the bids/ offers If you want a legitimate auction .

Also tap the blue @ at the bottom of my post to notify me that you have responded

In that case, how do I sell to other players?

you have to go to your pets profile click manage pet then set the price
if you want to sell to a certain player then you put there player id in the sell to id box

There are two ways

Direct sale: click on the item then hit the little envelope under the inventory box. Type in the players name and price. Click on send.

Broker sale: click on the item then hit the broker button. Type in the price you want ti sell it for then hit tye list button under the price box