If you aren't opposed to using an exploit then you can get infinite gold via such a method.
The exploit simply involves putting an item into the junk section of your inventory and heading to a merchant (you only want one item in your junk tab and you do not want the merchant to have anything in their junk section as otherwise the process gets slowed down).
So anyway you go to your merchant of choice and head to the junk tab, since you should only have one item in that section, this will go by much quicker.
Hit the Y Button first and followed very quickly by the A button, it will count as you having sold the item twice, by the item back from the merchant and repeat as many times as you feel like it.
"I love nailing Asari, so ageless and superior. Then you get them and they squeal like schoolgirls"- Gianna Parasini, Mass Effect 2.
Aside from the glitch mentioned above, the best way to make a lot of money is probably making lots of potent lyrium potions. You need someone with level 4 herbalism to make it though.
You can buy unlimited lyrium dust at the mage's tower, and the rest of the materials from the bartender in Denerim. The process can be a bit time consuming, but it's another option if you don't like exploiting glitches.
We were going full speed ahead... but facing backwards the whole time.