In order to be a functional merchant he must be a service class... i.e. trader service, thief service, etc. Once you've chosen the appropriate class and are done with his level and so forth, check and then uncheck the box that says "auto-calc", Then click on the AI button at the bottom of his page.
Another window will open giving you options there will be a service tab, click on that. You will see a number of options that describe things he will buy or sell if you check them. There is also a box that says "barter gold", just put the amount of gold you want him to have in there.
After you save it go back and be sure he has the stats you want. checking auto calc will reset his gold and all options you selected in the services window, so be careful about that.