Magic item prices would need to be revised. Sure, you could say that magic simply isn't for sale. But fact is, *anything* is for sale, given the correct price. "Not for sale" might simply translate into "much more valuable than the DMG prices would indicate".
So, assuming magic items *are* still present in your world, how much should they cost? If we stuck to the DMG market prices, no sane mage would prefer to craft an item himself, rather than buy it "on the market". That's because PC buying power increases exponentially with level, but XP only increases quadratically. So if a mage has to be twice as high in level in order to craft an item, at that point he'll have plenty of moolah to just buy one instead.
So the magic item market price should scale with the buying power of the crafter, otherwise he's not going to bother crafting one in the first place. Let's assume that in a standard D&D setting, the "average" magical item price is roughly proportional to the treasure value per encounter (I could have taken a fraction of the total wealth per level, but I only have the SRD at hand right now.) That means that PCs will be finding or crafting +1 items at around 3rd-4th level. Treasure per encounter increases exponentially with level, but if we compare treasure T1 at level L with treasure T2 level 2L, we see that the two scale approximately quadratically, with T2 = T1^2 / 400. I propose the new market prices should scale similarly.
For example, a 6th level PC in standard D&D will routinely be finding / buying / selling / crafting magical items worth around 2000 gp (6th level encounter value/encounter = 2000 gp). In the new "multiclassed spellcaster" system, a PC would have to be 12th level to craft these items. So a reasonable item market price should be around 2000^2 / 4000 = 10,000 gp (12th level encounter value/encounter = 9,800 gp).
Note that this calculation only depends on the internal consistency between market price and relative crafting cost. In particular, it is independent of the *utility* of the item at higher levels. For example, a low-level wand of magic missile will not be as useful in a 10th level part as in a 5th level party, so they may not want to pay as much for it. That simply means there will be less demand for such an expensive item than in standard D&D, but the price should still be as calculated above.