The text in the PHB doesn't prohibit the buying and selling of magic items in general. It pointing out that beyond a few common items it is a rare and special event. Pre-modern times had a similar issue in that there are items that could be bought and sold in a market but there were also luxury items so expensive that their manufacture and trade were a separate parallel to the normal everyday market.
Magic Items. Selling magic items is problematic.
This is true of all very high value rare items even in a campaign with a liberal policy of buying and selling magic items.
Finding someone to buy a potion or a scroll isn’t too hard, but other
items are out of the realm of most but the wealthiest nobles.
You can see this in the wage tables vs. the price of a healing potion.
A potion of healing costs 50 gp.
A skilled hireling, a craftsman, ect, makes 2 gp a day. Their monthly (30 days) salary is 60 gp per month. Buying a simple healing potion would cost nearly all of that person's monthly salary.
The price of a +1 weapons or +1 armor would be very high indeed from what we can see from the default economy. Make it a very high priced, rare luxury item.
Likewise, aside from a few common magic items, you won’t normally come
across magic items or spells to purchase.
Historically high value luxury items are:
- Commissioned to be made from a skilled artisan, in the case a wizard,
by a wealthy patron.
- An agent is hired to find and acquire a known item by a wealthy
- In rare cases in the largest cities (think London, Paris,
Constantinople, Beijing, etc) there will be merchants that
specializes in the acquisition of particular luxury items, again
magic items for our case, and arranges an auction for a known
clientèle. There is little in the way of "stock" and you have to be
part of the client network to be able even to bid when they do come
The value of magic is far beyond simple gold and should always be
treated as such.
As you can see from the historical example how high value luxury items are made and traded is different than normal markets. One additional comment that needs to be mention is that like royal regalia they will be considered prestige items. A mark of status as well as wealth. Of course there is their immense practical value.
You explained that your group is not greatly into simulation. In my opinion using the above options is perfectly compatible with that sentiment.
It opens some adventuring possibilities as the players can be hired as a agent to acquire magic items. If they do a good job, they will have the contacts to make their own buys.
Another adventuring possibility that the players are hired to aquire some needed components to complete a commission. Do a good job they gain an "in" to the network of wizards who focus on commissioning magic items.
Other than a baseline price guide, the most the referee will need to do is to come up with the odds of a particular item coming up. For example a +1 sword comes up for acquisition once every six months. For commissioned items, the referee will need to come up with chances of a wizard having some free time to work on an item.
With the just the PHB or Basic Rules the referee will need to come up with these values themselves. Hopefully with the Dungeon Master Guide there will be guidelines for the value of magic items. Considering the typical length of a Dungeons & Dragons campaign this should not be an issue until well after the release of the DMG in early November.
Once you've got the top level mundane gear and a stockpile of potions
(which won't take long), doesn't gold become sort of worthless?
Much of what I suggested involves an element of worldbuilding by the referee. If you are really adverse to this, and many are, then what you ask is a valid question.
My recommendation would be to spend it on hireling, retainers, and henchmen. How this different than your mention of maintaining a castle or a household. Because what you are doing is hiring in effect a small force to accompany you on your adventures. My advice to players for years that the best magic item is always another character. You multiply by the force of your group many times by hiring on. There are consequence namely in the logistics of feeding them and holding their loyalty. But given the rules as they are, as of Sept 2014, it is only "crunch" available to spend gold on.