Enforcing WBL is already part of the rules. The DMG brings it up repeatedly in 3 separate sections (maybe even more).

Quote Originally Posted by (MIC p233)
For many items, the market price equals the base price. For example, a cloak of elvenkind has a market price (and base price) of 2,500 gp. Making one costs 1,250 gp in raw materials plus 100 XP. Armor, shields, weapons, and items with a value independent of their magically enhanced properties add their item cost to the market price. The item cost does not influence the base price (which determines the cost of magic supplies and the experience point cost), but it does increase the final market price.
Same quote is in the DMG, but I'm too lazy to find it. The MIC cribs most of its rules text from the DMG anyways, and just updates the random treasure charts.

Quote Originally Posted by (DMG p199)
Table 5–1: Character Wealth by Level (page 135) shows the total value of a player character’s gear at a given level.
Quote Originally Posted by (DMG p135)
One of the ways in which you can maintain measurable control on PC power is by strictly monitoring their wealth, including their magic items. Table 5–1: Character Wealth by Level is based on average treasures found in average encounters compared with the experience points earned in those encounters. Using that information, you can determine how much wealth a character should have based on her level.
As we can see, "the total value of a player character's gear," is the measure of their Wealth by Level and market price/value is used when calculating Wealth by Level. Therefore, crafted items must be weighed at market value in order to appropriately measure a character's Wealth by Level. Simple.

The whole, "crafters can ignore WBL," thing is a myth. Of course, you're free to run your game anyway you want.

Here's another quote from a different section, just to keep reinforcing the point:

Quote Originally Posted by (DMG p13)
PCs and NPCs, victories and defeats, awards and afflictions, treasure found and treasure spent—all these aspects must be monitored to maintain balance. No one character should become significantly greater than the others.
If a character has twice the magic item value of another character, that one character is significantly greater than the others. So once again, crafting should not allow one to exceed WBL according to the DMG.

Still not on board? Here's an official "Ask Wizards" response to a question as to what the purpose of crafting was if it can't be used to get rich:
http://archive.wizards.com/default.a.../ask/20070303a

It took me maybe... five? minutes to find this. I'm sure you could find more if you really dug into the rules.