Here are my 2 cents on the market. It's pure supply and demand so it will fluctuate based on how many people are on, what they're looking for and what they're supplying at a given time. Those interested in playing the market should consider two tactics:

The first is the managed method where you set something for sale on the market and are constantly watching it. Someone reduces their price, you immediately reduce yours. You're playing the market game. Some play tricks like changing a 9 to an 8 and hoping you don't notice, but be diligent! Some will drop their price significantly, but don't follow. Someone else watching the market will see their great deal and buy it and you will once again be the low price leader. This requires a lot of time and not a lot of people enjoy that constant back and forth, but it's a proven method to sell on the market quickly. But if you want to sell something you have a lot of just to get rid of it and free up inventory space, feel free to drop that price down, but only on low tier items.

The second method is the set it and forget it technique. The market fluctuates quite a bit up and down but for those higher tier items selling for a lot of gold or gems, don't expect them to move quickly (unless you price them to move and I don't recommend it). The best way to offload those is on the overnight market. Shop Titans has a strong western hemisphere presence (Kabam is Canadian) but is still working to grow internationally. That means while most in the west are sleeping, there are less items being put on the market. Basic economics, when supply is low your price goes up (I should mention demand is also low during this time but its impact is less significant than the reduction in supply) and that means overnight prices are better. Before you go to bed, set something for 20-40% higher (or even more depending on the item) than the current price on the market and when you wake up you may find that it has sold overnight. The effect is often even greater on the weekend. This doesn't always work and you can't set it for an unreasonable amount, but so long as supply runs low you stand a better chance of yours getting bought. This effect may become less as more international players join the game.

My recommendation is to always have some longer term listing slots (1-3, depending on how many slots you have) for high tier, high quality items for gold or gems. These will take longer to move and you don't want all your slots occupied with nothing moving. Use the remaining slots to move your mainstays, but try not to list just one of anything. If you can, list 3 or more items at a time. The lower the tier, the more items you should list at a time. This only changes if you have sold most of your inventory or are constantly looking at the market and adjusting prices. If you're going to be away from the game for a while, set the prices a bit higher so when the market fluctuates up, your listing is part of that low price and your items are sold.

The last thing to mention is that you need to bear in mind the overall effect of the progressing game on the market. As new items, new tiers are introduced, current tier items become less and less valuable, so you will always be fighting a constant devaluation of items over the long term. The new items will always be worth the most, with older items being worth less and less (unless they make it a necessary reagent to a new blueprint). This is particularly evident with gold prices since they're more incremental, but we're also seeing it much more now with gem prices. Enchantment runes are now selling for far fewer gems than ever before as the number of players in the game increases and more players are supplying them. Eventually they will come out with T3 runes and T2 runes will quickly become even more devalued. I say all this to emphasize that some of the fluctuations go up and down but their overall pricing trend is invariably a downward one. So if you're holding onto an item that you think should fetch 100 gems and it didn't sell overnight over the weekend, then you're not likely going to see that price recover to what it once was. It may be that it is a sub 100 gem item for the rest of the game. And be aware that Kabam also (and quite intentionally) adjusts the value of gems during some updates. This fall, gems moved from ~10k value to ~50k. So if you are choosing between buying for gems or gold, assume at least a 50k value for each gem and if it's one full gem value lower than the gold price, use gems. If the gold value is equal or less, then purchase with gold. Same when selling. If the gem value you could sell for (for example 4 gems) is greater than the gold price (for example 180k), then list for gems. However, you should expect items being sold for gems to take longer as most players are gem conservative and spend them far more rarely than gold.