You need to have purchased the Bowerstone Old Town's weapons stall for this. If you haven't, you can do this now. Buy weapons on your own shops. As it is owned by you, the price should've been discounted by about 30%. Next, go to the blacksmith at Bowerstone market and sell the goods to him, the profit will most likely be higher then when you bought the items first. Repeatable as many times as necessary.
Alternatively, you can also go to Bowerstone Slums instead, and buy the most expensive weapon(s) there from the weapon trader. Next, go to Fairfax Garden, and sell them to one of the wandering merchant (one of them is a weapons trader). You will gain profit about 30-40% in return, as they bought higher than your expenses.
You can also start to get easy money at the time you have purchased your first shop. Every five minutes in-game, you will receive an amount of money. You can use this to your advantage by first saving the game, then changing the date of your Dashboard (make sure you aren't connected to XBOX Live). Change at least one year from today. When you have launched back the game, you will already have decent amount of money, due to the time skip. You can either do this again, or invest your money in other shops, and repeating the procedure. You will net more profit by doing that.