You can buy kitties from (or sell kitties to) your fellow cat lovers in the OpenSea Marketplace: https://opensea.io/assets/cryptokitties
Select "CryptoKitties" from the Categories on the left-side pane.
By default, ALL CryptoKitties will appear in the results window. Click the "For sale only" radio button on the left-side pane to filter only on kitties that are actually available for purchase.
At this point you have many options. Searching for a kitty by name is probably not going to be too useful. You can organize the card results with the sorting drop-down at the top-right area of the results window. If you're not sure which kitty traits you're looking for exactly, some helpful sorting options like "Lowest Price", "Most Traded", or "Recently Sold" might be of interest to you.
There are various filters for the types of kitties and their traits on the left-side pane of the marketplace window. These can help narrow down your search to find the perfect card!
Click on a kitty inside the search results to see additional details about it, such as it's price history, its parents, its children, its inherited traits, etc. Here you will also find the option to purchase the kitty if it's for sale.
Click on the "Buy" button once you have decided to purchase a kitty. This will take you to a new page with a more detailed description of the cost of the kitty.
Click "Buy Item". This should generate a Metamask transaction which you should SUBMIT in order to complete your purchase. Metamask should determine appropriate network costs, but you can choose your own if you want. NOTE: You have not purchased your card until you SUBMIT this transaction and it gets completed on the Ethereum network.
Once the Ethereum network confirms your transaction, you should be able to see your card both in the "My Kitties" section of the CryptoKitties website (https://www.cryptokitties.co/my-kitties) and in your OpenSea account (https://opensea.io/account) under "My Items".
Now that you know how to use the search features of the marketplace, you can use them to determine how much kitties are selling for that have similar looks/traits/generations to the kitty you want to sell. Once you have an idea of the price you're looking for, lets head to our OpenSea account page to setup our auction (https://opensea.io/account). If it's not already selected, click the "My Items" tab. Find the kitty you wish to sell and click on it. We're going to sell Coolio (the mother of Queenie from our breeding tutorial in the main page).
We'll click the 'Sell' button which brings up a page very similar to the Siring contract details. We need to select a starting price, ending price, and number of days to list Coolio for. Let's start at Ξ0.1, end at Ξ0.02, and list for 30 days. The price of Coolio will steadily drop over the course of 30 days. Coolio will cost Ξ0.1 at the time of this posting, and if Coolio were to sell in exactly 15 days, it would be for Ξ0.06.
Now that our parameters for the auction are set, we'll click 'Sell Item'. There will be additional steps to click, each generating a Metamask transaction with an associated cost.
First, We must click the button 'Approve Contract'. We must then SUBMIT the Metamask transaction that pops up.
Next we must click the button 'Create Auction', and again SUBMIT the Metamask transaction that pops up.
Metamask should determine appropriate network costs, but you can choose your own if you want. NOTE: You have not completely created your auction until you SUBMIT both transactions and both are completed on the Ethereum network.
Once they are complete, we can view our auction at https://opensea.io/account and clicking on the 'My Auctions' tab.
A Kitty's rarity depends on a variety of factors. Check out https://cryptokittydex.com/cattributes to get an idea for which traits are the rarest. OpenSea will be providing detailed Breeding and Siring guides soon! Keep checking back.