Guides

-This thread is not for buying or selling dragons-
The auction house is a beautiful thing, whether buying or selling. For sellers, however, it can be a bit tricky to figure these things out.

-The first thing to do is check to see how much an item will cost if you directly sell it from your hoard. Typically, AH prices will be higher- but definitely check!!
If an item sells for 1500 directly out of your hoard, and the lowest AH price is 1575, don't waste your time- putting it up for auction won't return enough treasure to make it worthwhile.
Also, keep in mind the courier's fee. Prices for courier's fees can change depending on how long an item is up for auction Please feel free to double check this fact, as its only a hunch at the moment

-Next, realize that the AH automatically puts items in order by time of expiration- the first auctions to expire will be the first things you see. HOWEVER those prices are almost NEVER the best (as the lower prices are few and far between)
Even if you see an item up for 25000, it is most important to search the item in that cute little search box to the right, then change the filter setting to sort by price. To make it easier on me, when I need a treasure item, I also add '0' as the starting price next to treasure, just to filter out the gem prices.

-For buyers, you can stop here! Once you find a lower price (feel free to check the marketplace as well, but the AH usually has better luck) click that button and reap your rewards! Doing this can end up saving you A WHOLE BUNCH OF MONEY.

-Sellers, go ahead and do the same thing as the buyers- in the search, put the item you'd like to sell, then sort prices from low to high. This is very important, because it serves as a comparison to the marketplace price. Again, if there isn't a significant price difference, go ahead and sell it directly from your hoard. If not, then set up that auction!
The best way to profit, I've found, is to put your price slightly below the lowest offer's. If they're asking for 15000, ask for 14750. It may not seem like much of a difference (and honestly, it isn't) but most buyers will look at the price comparison and pick the absolute lowest price- it is just an item after all!
The next thing to do is sit back and be patient c:

Some More Tips:
Monitor the AH or some buying threads for patterns, and hold off on selling an item until demand is higher.
Buy all festival items. Especially if you don't want them. Then, wait about a week after its over, and do all the price checks. This is the best time to sell the items, because everyone realizes that they want it, or they didn't have enough festival currency to get it at this time. Also, its before most buyers give up searching for the item.
If selling an item doesn't make much profit at all, send it to a donation thread! Check newbie threads to see if they need any help with anything, and do your best to assist. I usually just donate mine to freebie or swap threads, and call it a day.
I always sell skins. Mostly because selling skins is very profitable, and partially because I don't like most of them :,]

Trading Post
Some items just won't sell for much. Keep them! Swipp may have a deal for you on particular items (which isn't mentioned in this post, but there's dozens of others!) and Crim is whimsical and will probably ask for it eventually.

Did I miss anything? Have anything else to add? Please let me know!

As a little side note, I made 250k in the auction house in less than 24 hours using this method.
Another way to benefit is to buy cheap items (like drastically cheaper than the other prices) and turn around and sell them for a more fitting one. I personally don't like this method, but some people say they profit from it.