You do it via a process of investment, and then using profits obtained from the investments to repeat the process over and over. It's more complicated then that as you need to be able to read the market to some degree b/c prices fluctuate as does item desirability. You also need to know when to buy in to specific bulk items, and when to sell out.
The important part is to maximize your profit at all times, but to remember that items sitting on your vendors for weeks at a time is money you don't have access too. The method I follow is that you buy low, and you sell low, if you have great prices, and you're the cheapest around, or close to it people will remember and they will make sure to visit your vendors first to see what you have.
The most important part is that you can't sell gold in small clumps if you ever wish to be considered truly UO wealthy. You have to acquire a significant sum before you start selling it, generally around 1-2 bil, although there is some flexibility there. If you start selling gold to early though, it may take awhile to build up a reputation and gain buyers, but if you do not have enough capital to support stocking your vendors and to sell to your clients youll find yourself very poor very fast.
