You can buy cards from, and sell cards to your fellow gamers in the OpenSea Marketplace: https://opensea.io/assets/mythereum
Select "Mythereum" from the Categories on the left-side pane.
By default, ALL Mythereum cards will appear in the results window. Click the "For sale only" radio button on the left-side pane to filter only on cards that are actually available for purchase.
At this point you have a few options. If you know what card you are looking for, you can type its name in the search bar above the results window and hit ENTER. 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 what cards 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 cards and their abilities on the left-side pane of the marketplace window. These can help narrow down your search to find the perfect card!
Click on a card inside the search results to see additional details about it, such as it's price history, card number in the set, stats and abilities, and more. Here you will also find the option to purchase the card if it's for sale.
Click on the "Buy" button once you have decided to purchase a card. This will take you to a new page with a more detailed description of the cost of the card.
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 Cards" section of the Mythereum website (https://www.mythereum.io/#/my-cards) 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 similar Mythereum cards are selling for. 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 card you wish to sell and click on it. We're going to sell our Card (Mythereum #921).
We'll click the 'Sell' button which brings up a page for us to set auction parameters. We need to select a starting price, ending price, and number of days to list our Nyx card for. Let's start at Ξ0.12, end at Ξ0.08, and list for 30 days. The price of our card will steadily drop over the course of 30 days. It will cost Ξ0.12 at the time of this posting, and if it were to sell in exactly 15 days, it would be for Ξ0.10.
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.
Card rarity is not yet implemented, but you can check the relative rarity of a card by checking how many of that card exist compared to the total number of cards created. OpenSea has created a helpful chart showing the current statistical rarity of the existing card set. Navigate to the Card List and Rarity page.