I commented that I've seen one work-around for the magic item shops vs the domain game dichotomy: powerful magic items are only traded with non-mundane currency (astral diamonds, for instance), so after a certain point, a pile of gold can only be spent to upgrade your castle because it can't buy a better magic item. Meanwhile, any found astral diamonds can't be used to upgrade your castle, because they're only used for buying magic items.
I'm already on record as disliking how magic items work in GURPS Dungeon Fantasy: the utility of any random item is generally pretty low compared to its sale price, so everything turns into vendor trash so that the PCs can buy an actually useful magic item or better mundane equipment. Now I'm wondering if astral diamonds might be a partial solution to the issue. I'm going to noodle around with idea and maybe someone will comment on it.
Dungeon Fantasy doesn't allow PC enchanters, and makes the rules for enchanting vague since PCs can't access them. So I'm going to rule that every enchanter can only make one powerful magic item every decade or so, unless he has access to astral diamonds: astral diamonds can be converted in powerful magic items at some rate, and powerful magic items can be converted into astral diamonds at some rate. Most powerful magic items are exchanged for astral diamonds, and vice versa, though sometimes you'll find someone with way more money from sense that will buy a powerful magic item for a ridiculous amount of gold (which is how enchanters get the gold they need to pay the mortgage and buy dinner), but people with many magic items rarely or never buy them with gold: it's astral diamonds or get out.
Only Astral Diamonds Here
This means that PCs delve, searching for powerful magic items, astral diamonds, and gold. Gold can buy mundane gear, castles, and minions. Powerful magic items and astral diamonds can be traded for each other, but not for gold, so there's no point in trying to trade a Puissance +1 sword for a suit of fine plate armor. Gold can, in theory, be traded for powerful magic items, but the exchange ratio is so awful that delvers will rarely do so: PCs are generally kind of reluctant to pay $10000 for a Puissance +1 sword, there's no way they're going to pay 100x that amount especially when they have a stack of otherwise worthless astral diamonds that could be used to make the trade.