What is the most effective way to "play" the auction house?

Here are some tips based on my experience.

Buying Low

• Know your prices. Sites like Hexprice aggregate real data from the auction house to give a constantly updating picture of trades that are occurring. If you see something for sale well below the median price over the last few weeks, it should be a safe buy. Keep in mind that prices can change fast and the data is always two days behind.
• Understand sniping. Since Hex doesn't auto-increment prices, bargain shoppers should either use buyouts, or the minimum bid. If it is a true bargain, the only way you'll win a bidding war is if you bid last. Raising the price prematurely just eats into your profit margin.
• Sort by time remaining. Pick a relatively large set of cards (an entire set, color, or rarity) and click the time remaining column to sort. While you won't see the actual time remaining in each auction, the underlying sorting still takes it into account, so the first pages will all be ending very soon. The smaller the time remaining, the more likely you will be able to be the last bidder on the item.
• Buy containers. Both chests and packs (especially primals) often sell for less than the expected value of their contents. If you see one at a low price, grab it, open it, and list the contents. You won't always come out ahead, but over the long run you definitely can. Note that Hexprice calculates the expected value of a primal pack for each set, but it doesn't include the guaranteed legendary chest, which is worth roughly the same as another rare card.

Selling High

• Check existing auctions. Even with Hexprice, it is important to know what items are currently listed for. If there are other auctions that are cheaper than yours, your item is unlikely to sell. If there are too many cheap auctions right now, hold your item for a bit until they clear.
• Use buyouts. You want to sell to collectors, not other bargain hunters. Listing an item without a buyout means it will almost always go to a sniper for less than the real value. Set the minimum bid at a price that you'd be OK with, and the buyout slightly higher but ideally lower than any existing ones.
• Keep the duration short. I almost always use 24 hours rather than 48. The extra 24 hours doesn't net you much in terms of potential buyers, and if you list for 48 hours, other auctions are likely to undercut you before yours finishes.
• Watch for "over pricing". Some rare items are in low supply, and the only auctions listed will be well above what the historical data suggests. You can take advantage by undercutting significantly, but at a price still above the historical data. Searchers may interpret your listing as a bargain based on the other listings, even though you're actually marking it up. It can also be helpful to make these auctions long. If people are trying to artificially pump the prices of items, they will be forced to buy your listing at the marked up price in order to hide their scheme!
• Use gold to avoid price minimums. The auction hours enforces minimum prices on platinum sales. To get around this, sell for gold instead. It is relatively easy to convert between the two currencies, and lots of small gold sales can add up quickly. The conversion rate has historically been around 100 gold to 1 platinum, but Hexprice can help you track fluctuations. The home page lists the currently calculated rate.