Hey there - I spend nearly as much time merching in SK as playing the game itself, so hopefully I'll be able to provide some useful pointers.

There are three main aspects of understanding which separate a bad merch from a successful one - items, market trends, and personality

Prices are probably the only factor that will feel important when you're first starting off - learn the prices of items, and buy for less than you resell for. Simple enough, right? There's just one catch though - you aren't trying to make the most money possible in one sale, you're trying to make the most money for your time. When considering if you should buy an item to resell, you'll want to know its price and how sellable it is. If you're only making 10k cr from reselling an item, it'd better be something that doesn't take much effort to sell. If you're making 1m+, on the other hand, it's okay if that item is going to take a while to sell. You'll start to learn the prices/sellability of items as you go, but watching the auction house for a while and seeing what sells/how quickly should give you a rough idea of what's going to be worth your time.

It's important to understand that psychology plays a huge role in SK's market because of how small the playerbase is. Some items will sell faster/sell for more on the auction house than they will on the forums or trade chat (even after the 10% AH fee!), and vice-versa. The items that sell for more on AH tend to be items which are high in demand and low in price (such as enamorocks/low-tier accessories) or the lower end of high-tier items (things such as wolver tails other than polar/iron, basic/volc/div/shadow dragon/valk wings, and relatively cheap rares such as prismatic glow eyes). Ones that sell for more on trade chat and the forums tend to be either low in both supply and demand (such as leafy aura) or high in demand but high in price (such as dread aura). If you decide to sell and item in trade chat/forums, you'll want to consider that some items will sell more easily if you price them as crowns instead of the equivalent ce price, and vice-versa. As a rule of thumb that'll hold true 90% of the time, if an item's going to be more than 400k you should try selling it for the ce equivalent. Between 200-400k either will work. Below 200k, sell for crowns rather than ce.

Once you start to understand why some items sell more easily with different currencies/venues, you'll be ready to take a few risks - when new items come out, use your understanding of the market to predict prices. New prize boxes/monthly lockboxes are some of the most obvious examples of easy profit, provided you understand item prices. A new monthly lockbox's low tier accessories (such as binoculars in the copper box) will be able to sell for 200k+ on day 1, and lower to 40-50k cr by the end of the first week. A new promo box's 1% tier item (such as venom aura) may sell for 100k ce on the first day, and 40k ce by the end of the week. If you see someone selling that same aura for 60k ce on the first day, you can buy and resell it for massive profit. If you don't know what you're doing and pay 110k ce or buy the aura for 60k on the second day instead, you'll end up losing money - only go for newly released items when you're confident in your prediction abilities.

Personality does matter. Buy low/sell high only holds true to a point - if you severely underpay someone for their items, they'll probably never trade with you again after learning what the items were worth. If you make fair offers, you'll gain trust and people will come back to trade you again. The time that'll save you when buying/selling in the long run is well worth the smaller profit margins. Selling too high doesn't make sense either, as overpricing items just makes them take longer to sell. There are exceptions to these rules, of course. If someone offers to sell you their item for a low price, it's fair to take them up on that offer. If you know an item will sell for far more than it's worth on the auction house, go ahead and overprice it (items such as orbs of alchemy tend to sell for significantly more than they're worth if you list them on AH)! You won't lose any time since the AH is automated anyways.

Some easy items to sell for profit when you first start out are heart pendants, orbs of alchemy, sparks of life, and various 2-3* craftable gear. You can make/buy these in supply depot yourself, and will be able to make 100k+ daily just listing these on AH. By the time you've made enough to buy/sell items with other players, you'll have hopefully learned some items prices just by browsing through the AH.

Feel free to mail me in-game if you've got any questions/need clarification on anything here. Good luck!