You can buy Fighters from (or sell Fighters to) your fellow CryptoFighters Fight Masters in the OpenSea Marketplace: https://opensea.io/assets/cryptofighters
Select "CryptoFighters" from the Categories on the left-side pane.
By default, ALL CryptoFighters will appear in the results window. Click the "For sale only" radio button on the left-side pane to filter only on Fighters that are actually available for purchase.
At this point you have many options. 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 Fighter traits you're looking for exactly, some helpful sorting options like "Lowest Price", "Most Traded", or "Recently Sold" might be of interest to you. Gen 0 cards will tend to be the most desirable, though it's possible lower tier cards with rare (or very many) traits could also become sought after as well.
There are various filters for the types of Fighters 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 Fighter inside the search results to see additional details about it, such as it's price history, its inherited traits, it's generation/level, etc. Here you will also find the option to purchase the Fighter if it's for sale.
Click on the "Buy" button once you have decided to purchase a Fighter. This will take you to a new page with a more detailed description of the cost of the Fighter.
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 Fighters" section of the CryptoFighters website (https://www.cryptoFighters.co/my-Fighters) 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 Fighters are selling for that have similar looks/traits/generations to the Fighter 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. Select one of your Fighters to sell and click it to open up it's OpenSea asset page.
We'll click the 'Sell' button, and then we'll need to select a starting price, ending price, and number of days to list our Fighter for. Let's start at Ξ0.014, end at Ξ0.01, and list for 30 days. The price will steadily drop over the course of 30 days. Our Fighter will cost Ξ0.014 at the time of this posting, and if it were to sell in exactly 15 days, it would be for Ξ0.012.
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 Fighter's rarity depends on a variety of factors. For now, the things you want to look for are the Fighter's generation (Gen 0 is the best) and the Fighter's traits (some traits are rarer than others, but having MANY traits is also really good)