Mule Character Achievement
Purchase every inventory upgrade.Inventory upgrades can be purchased at the blacksmith in Brighthoof after completing the third story quest, 'A Hard Day's Knight.' Purchasing them all will require a huge amount of gold - approximately $300,000,000 to afford them all. Even with natural gameplay coupled with farming and grinding loot to sell, you'd be looking at dozens of hours of purely farming bosses to grind out loot you can sell for money. Really, the only way to get enough money in a reasonable amount of time is to take advantage of this game's version of inventory and money duplication. This requires a second person online, but will get you hundreds of millions of gold in not much time. Full credit to Cheetah for the video below.
If you can't or don't want to use this method, unfortunately you don't have any faster option aside from farming a boss over and over, looting all the items it drops, selling them all, and repeating. You'll likely end up with around 3-4 million gold per hour depending on a few things, so you're looking at a huge grind to be able to afford everything.