He uses the proccedes from magic item creation to hire adventueres to bring back mosters to his labratory. In each case, the box has a small hole or slit in the side with a label under the hole indicating the name of the creature and a phrase that says "Insert Sword here for XXX Experience Points."...
On a more serious note, your question is a good one and worth some discussion.
For the most part, I usually assume that the creator is able to use some sort of "Power Component" that can be purchased from local adventurers/magical supply shops that allow him to spend 0 or almost 0 XP when constructing magical items. The cost of these components reduces his profit margin from 50% to about 10%.
ie. If a +1 sword has a market value of about 2000 gp + the swords cost, and the rules say that this costs the enchanter 1000gp in materials and 80xp, then I assume that the enchanter can compensate for that 80xp in other ways simply by purchasing other "power materials" worth another 900gp.
Page 96 of the 3.0 DMG talks breifly about power components and suggests a cost of 10 to 20 times that XP cost. I assume that the NPC enchanter, based on the volume that he deals in can usually get the lower end of this scale when maing magical items.
Of course, at the same time, this may not always be possible, nor can it be assumed that the NPC will always be "Adventure-Less" He will have the occational break in or attempted robery, or when he travels to the next town to visit his brother, he may be attacked by highway men.
Then there is always the question of how did he get to the level nessecary to be making magical items in the first place. Was he an adventuerer at some point in his past, or....
Hope this info helps
