The secrets of success of a merchant in circa 1200 AD Europe is much the same as that of an "import/export business" now - buy low, sell high. Its simple in principle, but it's the "how" that makes for a good story.
Work The Supply Chain
A classic example is getting to know people who don't realize the true value of what they have. Let's say you get invited to a dinner party where you taste some really good cured meat appetizers. You politely gain a reputation as a pleasured guest by complimenting your host, but you ply the chef with your charm and find out who he bought it from.
With some leg work you find a butcher that humbly sells his goods for the usual going price of meat, even though the work and craftsmanship put into the curing process is truly expert. And he just sells it for regular prices and even gives it away as the occasional gift to local nobility!
You know very well that what he sells for a coin you could easily sell for 10 or 20 to the kitchen staff of fine estates. So you strike a deal to acquire these fine 'gifts' for your friends out in the country, and what you bought for 100 you can easily sell for 1000 to your 'contacts'.
Expand Your Operations
If you develop enough buyers in enough quantity, you can perhaps strike a deal with the butcher that in exchange for hiring people to help him and helping to secure the finances for a larger cure-house, you guarantee you'll purchase some quantity of goods for just as good a price as you are already giving him.
If that goes well, your reputation for providing a fine set of cured meats to nobility draws the attention of a bon vivant prince with money to burn and extravagant tastes - perhaps he likes more than just good food and drink? Perhaps he wants more than just a taste of Arabic hashish, but there's some exotic poisons/books/paintings that just so happen to be open for the taking in such a market? A man of his status can't send his own agents...but perhaps you know how to be discreet?
In short, one satisfied customer leads to more sources and more customers, and your ability to make money as a merchant is about finding not just individual opportunities, but spotting stable trends you can take advantage of. Once your meat operation is up and running you have other people doing the day to day chores, and you can move on to bigger and better schemes.
Enhance Your Profits (and Other People's, Too)
Buying silver from the mines at market rate can be profitable if you sell them far away, but when you are getting bars under the table from a coin mint in exchange for silver bars mingled with cheap metals you managed to put together (plus bribes, of course)? And what if its all being underwritten by the nobility on top of it all, they just need not to be seen as being involved?
Cheating Makes The World Go Round
You make a little money buying something for a penny and selling it for two. But to get filthy rich you need exclusive access - ways to cut down on prices and increase the final sell price. So you can be honest and helpful and anticipate peoples need (like a famine looms and you ride to the rescue with whole caravans packed with food), or you work the 'gray area' by taking advantage of situations...or you just do anything to turn a profit, because hey, people plagued by a mysterious disease aren't in a position negotiate the price of your cure, are they?
These are all suggestions with no magic involved at all. Magic makes everything better! Faster transport, lighter burdens, summoned materials, superior materials, blacksmithing goes a lot faster when you have fire spells to run that furnace and magic hammers to slam that stuff out flat in a hurry, you can convince people to sell only to you, invisibility makes smuggling a lot easier, etc. I think that's all the 'easy' part - but if there's no profit without magic then magic probably is just a waste!
Further Reading Material
"The Richest Man In Babylon" was written as a guide to how to manage your own finances in the present day, but it's set in the ancient era, and its full of good examples of how to do things right and how to do them wrong (trusting a mason to buy rubies, for example, can get you sacks full of worthless trinkets). Medieval Europe wasn't much different - buy low, sell high. It's also just a nice book.
"Against The Gods: The Amazing History of Risk" is a great book about risk management, and often details some of the finer points of how commerce was handled throughout the ages, including the origin of insurance "underwriters" and so much more.
"The Merchant of Venice" by Shakespeare is a nice dramatic look at the mental and social life of upper class merchants from a later period.
Note that historically the rise of the merchant class is generally later than 1200AD, and so the independence from local nobility would be unusual (though not necessarily impossible). Realistically you'd be dealing less with fellow merchants than with aristocrats and nobles, and according to this: http://www.euratlas.net/history/europe/1200/index.html
...you'll be dealing with considerable religious tension from the conquering Islamic Almohads in south/central Spain. That means more opportunity for profit, and for getting in trouble. Lots of potentials for naval involvement, and realize that European powers were no where near total naval dominance of the Mediterranean, but the Atlantic of western France was I think pretty safe from Islamic incursions.