Although Ultima Online's economy was player made, most of the money came from adventurers, especially the tamers and bards. Tamers and bards or tamers/bards were capable of farming the hardest creatures in the game which were dragons, ancient wyrms, and abyss creatures. These creatures not only dropped gold, but they also dropped rare items, increasing the potential value of goods that these adventurers were capable of receiving.
The crafters on the other hand were probably the next richest players in the game. If they kept their vendors in stock chances are many players would purchase goods from their vendors. However, UO economy had a few issues that kept even the best crafters from making it big money such as tamers. The problem with the UO economy was that gear although was lost by players, it was rarely destroyed. Most of the time gear just transferred from one player to another, which meant that if a player lost his/her gear to someone else the supply of good did not decrease or increase, it just changed possession.
This meant that the overall demand and supply of goods did not change often because most of the gear was still circulating in the economy, they were just shifting between players. Not to mention, there were magical drops that were better than crafted gear, and although this did not occur very often, there were quite a few of them.
Many players who played UO think that it had the best MMORPG crafting economy. I think it was one of the best, probably the 2nd or 3rd best crafting economy I experienced. I believe Eve Online and FFXI did a much better job with its crafting economy. But nonetheless, Ultima Online did get a few things right which is why its crafting economy thrived. UO managed to create a barrrier of entry to crafting. They did this by raising the difficulty of becoming a GM crafter and for most the money was made by being a GM crafter and nothing less.
Another success UO did that enhanced the player economy was the content. UO had a lot of player made content that was available. Players didn't have to choose crafting or pve/pvp. Like I said, a player could of been a thief, a beggar, a tamer, a pirate, a treasure hunter, a bard, a mage, a warrior, so many different things that players were not limited to just being a warrior/mage/archer and pvp/pve or craft. Hell some players were home designers spending hours decorating homes and/or selling them. The game design and content was vast so it kept those who wanted or needed to be a crafter to a minimum, it wasn't as if you had to be a crafter to progress or to have something to do (like in a lot MMO games I know).
Not to mention crafting wasn't a subsidiary or an after thought, it was a stand alone and important part of the game. Crafting in Ultima Online was just as important to the pve and pvp, if the crafting economy had failed to delievery goods to players, UO would have died many eons ago.