Auctions are the only way for players to sell their items, but instead of in a fixed price; players can "Outbid" the previous price, Making the Price for the item(s) higher than normal.
However you can only sell consumables in a fixed quantity : 5,10,30 to 50

Basically you can use Auctions to sell your items and if you're lucky you may even get Twice or even Thrice of the default value! in this guide there will be "Tricks" to make players into bidding more than they should, and also tips that will show you how to Buy items in a low price.

First! you are recommended to sell items that you have in LARGE quantities, to give the illusion that the silver they are bidding is worth the item they're buying.
E.g : 5 Ointments are usually bought for 50-100 silver, 10 Ointments are bought for 100-300 silver, see the difference?
But it usually depends on the mood of the players in auction and also the items in circulation.

NOTE : Consumables usually sell higher than Equippables items in an "old" server, since most players are well geared.

So if you are in a server that just started (2-5 months ago), you can sell your gear for an amount more than the default silver value, and for "Old" servers; Artworks, and other Consumables sell higher. The value of an item also rise and drop, keep that in mind.

Second! Just as the NOTE above stated "Consumables usually sell higher than Equippable items". Artworks, Scrolls and Buckets are your recommended item to sell (if you don't want to use it). they sell at very high prices 1000-7000 silver.

Third! you will have to know when to back out of an Auction, your silver won't be spent until the timer runs out, and you can't back out once you are the highest bidder; if you bid 1000 silver for 5 Ointments; you will spend 1000 silver on 5 ointments (which is over-kill and should not be done).

When an item's price is going higher than the price you are willing to spend, and you don't particularly need the item you are bidding it is best to back away.

And a trick to Outbidding; since you don't know how much the highest bidder is bidding and you don't want to bid way over what the highest bidder bid, it is recommended that you raise your bid gradually by 10-200 step by step (depends on the item being bid). More details on outbidding here.

If you are being chased by the timer, best to back off.

Fourth, from personal Experience; I once bought a Book of Wisdom for 500 silver and sold it back for 1500 silver.

So if you see an Item being sold for a Lower than default price BUY IT! and then if you are lucky you can sell the item for more than when you previously bought it. so you can essentially be a merchant in an Auction and... "Buy low, and sell high"

And lastly you can ask around and see what players on the server are doing, if there is a war going around or failed raids, and you can use these events to your advantage and sell items that would usually be cheap with a high price! But it is difficult to time the item for sell perfectly (since there is a delay before items go on auction).

 Conclusion : to make it big in auctions, it is all up to your wisdom and luck, but Practicing and taking notes on how much players are willing to pay for items is advised; so you will know when an items value is rising or not.