Goods Auction PhaseNow is the chance for you to profit off your hard work, and to prepare for the following rounds. The Goods Auction Phase is where you will get an opportunity to buy, sell, and/or trade the goods you've produced.
There are four auctions each round, one for each resource (in order: Smithore, Crystite, Food, and then Energy, displayed as a sign in the upper left). The center of the screen is where players will raise and lower their asking prices, in an attempt to meet (either with each other or the Store) and exchange money for units of goods.
In the upper right and lower left corners, the Store shows the price at which it is willing to buy goods from you (lower left), and the price at which it will sell goods to you (upper right). You can also see the number of units the Store has in stock; If the upper-right corner shows nothing and is dimmed, this means the Store is empty. Bear in mind that the Store is in the business of making a profit just like everyone else, and adjusts its prices throughout the game based on the available supply and demand of goods, and the average price that the good is trading at (whether between players or with the Store itself).
BEFORE THE AUCTIONFor each auction, you see a colored bar showing what you started the round with, what was used during development/production ("Usage"), how much spoiled/decayed ("Spoilage"), and how much was produced this round ("Production").
A "critical line" then appears, indicating if you have a projected shortage or surplus for the next round (except for Smithore/Crystite, neither of which is "critical" for survival). The Food line always shows exactly how much Food you need for a full time-bar, and the Energy line shows how many units you will need if you add/convert one more non-Energy MULE to your plots (if possible). The exact amount of your projected surplus/shortage is shown in the display below your column.
DECLARING YOUR POSITIONYou have a few seconds to decide whether you will be a Buyer or a Seller in the auction. Use the up/down controls to put yourself at the top to be a Seller, or the bottom to be a Buyer (the game will put you as a Seller by default, if you have a surplus, but you can switch). Once the declare timer runs out, you're stuck in that position, and the auction will begin in less than a second!
FAQ: When should I start holding up/down to move during an auction? I always seem to be behind.As long as the countdown is still visible, you are able to change your position as Buyer or Seller. Once the countdown disappears and the buy/sell lines are visible, you are free to hold UP (as Buyer) or DOWN (as Seller), and you'll be assured of not missing a step when trading starts.
THE AUCTION BEGINSSellers move down to lower their asking prices, and buyers move up to raise their bids. The dotted line shows both the lowest price offered, and the highest bid among all four players. All characters can move at the same time, trying to reach an agreement before the auction time-bar runs out (but it moves slower while players are moving, and pauses when trade occurs).
When a Buy Line and a Sell Line meet, units and cash start to trade hands, which you can see by looking at the shortage/surplus displays and the cash totals at the bottom of the screen. Either player can stop the trade at any time by moving away from the line; otherwise, trade continues until the Seller runs out of surplus, or the Buyer runs out of money (either will cause the affected trader to "snap back" to the top or bottom of the screen).
You can always buy from the Store (if they have stock) by moving to the top line, and sell to it by moving to the bottom line (unless aggressive trading has raised the price range beyond the Store's buying price).
FAQ: Why did my character suddenly appear back at the top after selling Food/Energy? I had more to sell.The game assumes that you never want to have a shortage of Energy or Food, and snaps you back when you hit your "critical level". However, you are able to move back down once you've reached that "critical level", and sell more, at your own peril. (There are times where it's completely justified to do this...see some of the forum articles on Advanced play for info.)
FAQ: I got to the trading line right as/just before my opponent did; why did he get to trade first?There is a small amount of lag compensation/lag forgiveness that gets factored into who reaches the trade line first; that being said, the general rule is that when there is a tie at the line, the players furthest behind get to trade first (the "1st Place Curse" strikes again!).
FAQ: Yes, but can't I trade just one unit?Once trade between two entities starts, no one else can break it. You must convince one of the players to stop trading, in order for you to get in.
FAQ: The Store is out, and I keep moving up to buy but I can't reach my opponent.This is what MULE veterans call "testing the market", but you might consider it "price-gouging". When the Store has no units of Food or Energy, players who happen to have a surplus amount can wait at the top of the screen where you can't reach, until you drive the price up to a level they are willing to trade at. Then, if they're nice, they might come down to trade with you.

IMPROVE: Instead of a short delay between the Production and Auction phases, require all players to press their action button.IMPROVE: When production bars rise, constantly update the totals at the bottom instead of waiting until the bars have stopped.IMPROVE: To remove needless delay in the game, when a situation exists where players can no longer buy units from other players/Store, or no longer sell units to players/Store, or all players hit some special command to indicate they are done trading, end the auction immediately (after a brief moment).SPEC: In the original game, players moved much slower in the auctions; you could easily plan your pricing down to the exact dollar amount. In Planet MULE, things move much faster and it's easier to lose control. This is especially noticeable on buying plots of land, where you end up paying way more than necessary.SPEC: The original game had a special feature during auctions called collusion, which allowed 2 players to trade goods privately among themselves without interference from other opponents. This provided a means for players to work together and lock out a leader from a buy-out situation, or purposefully exclude him from trade even if he had plenty of cash/goods to trade with. The developers have not stated for the record whether they intend to return this feature to Planet MULE, but have indicated that there are difficulties in enabling it for internet play.