Foreword, haven't played since a long time, though I plan to. Still undecided whether I'll use a mix of spycraft/mecha compendium/fields of blood/elements of magic mythic earth or pathfinder (I stayed with 3.0)
I'll prefer to use the term trade instead of economics. (Some things can break trade, nothing can break economics as it is the study of trade).
So to add oil on the fire (something else to the argument), when a spellcaster launch fabricate, he doesn't get something for nothing.
He did learn the spell, which took time, and took time to cast it.
Think writers (of RPG books for example): to attain their level they put work, when they write the book it doesn't cost them materially, though the PDF has some value. Now depending on the laws, the money they get might have been spent upfront (medieval) or for each copy (modern IP law).
The wizard fits perfectly in this model.
I think that irking points are the recommended amount of magical items based on level and fixed price list.
I don't want to break the recommended amount, because it's quite useful (I did apply strictly the treasure table in ADnD 2nd edition... with characters who attained level 12... gazillions of gems work of arts and +1 swords)
Players wants trinket, having a form of power level of items through gp and treasure table that fits the encounter level is a feature for the GM.
So how to deal with player who trade skills/items/whatever?
Why not give them XP?
You keep the standard items price list, including the one for magic items. You just change the spell for hire price list.
You had sellsword who got money for overcoming challenge by killing things, with standardized amount using treasure to compensate the meager offer, or no treasures and higher bid. Now you get sellspell.
What is missing is someone with the patience to produce a chart which ensure that the xp and money gained will fit the recommended level for a given amount of money. (and an easy formula for the spell price based on spell level and caster level for spell with variable elements). ;)
It keeps working with the various assumption made in previous adventures modules. i.e. no downtimes because the adventure is pushed on the players. And it doesn't penalize groups who choose a different path (having a ship builder, and the building of a ship as an integral part of the campaign).
NPC didn't gain xp, though if you want a rationale: mundane works does not give xp (so the forger doesn't necessarily improve), though if the character overcome challenge (or try to improve his trade), he gains levels.
It allows solo adventures for forger, thief and the like.
On trade. The fixed list is nice, but it exclude trade. If everything has the same price everywhere, there is no trade.
So, keep the fixed price list in the core book, and suggest that city or hamlet have a few different prices (for example, provides an hamlet with cheaper vegetable). It would gives the idea that there isn't an infinite buying or selling power, and that prices change without introducing bookeeping. It might be easier to justify that once every one has a +1 sword, the price of the +1 sword is 0.
The idea is to simply allows PCs to influence trade (better tax return if the monster are killed on the road, so the queen has a reason to hire adventurer). It's part of the game: having choices.
Now, whether gold is a good currency depends on it existing in finite amount. The game seems to assume it's the case, if needed require gold piece to be minted (and some weigh of gp doesn't has the same value for the same weight in gold). If the game introduce insane amount of gold from dragon hoards, just assume that some of it is transformed in statue or the like. (though, a platinum, or whatever currency in finite amount, at higher level would be useful).
Last if the low level king heir sell his castle to the adventuring party for their magic items, he won't stay alive long (nicely explain what kind of lame adventuring party died in a monster lair leaving magic items which should have allowed to kill the monster). If the party decide to sell their magic items for a castle, so be it (somewhere it means that the castle is a tools for overcoming their challenges).
There is certainly loopholes left (depending on the separation between wealth and gears, solution like Rokugan where items builds up or spycraft the big score might help), though, I think that this one allow to keep all spells, magic items and their price.
Results: just gives xp for gold if it's part of the overcoming of a challenge (found someone who wanted you to build a sword, steal the king crown).
P.S. I shouldn't try to write anything at 3 a.m.... Someone to tell me how this change wouldn't help solve the problem? (or if I completly missed the problem?)