The economic model makes sense due to the fact that you're selling things that, well, not everyone is going to want and a shop owner is going to need to hold onto an item for some time. Figure that 2k of gold + cost of a masterwork weapon. Now take that into a severely-underpriced modern framework. I have a light easily-transferable lump of steel with magic on it that could rake in $100k in open market for sale in real-world money. I have to protect my shop from a group of adventurers who hire 5-10 strong-backed commoners who come in, kill my defenses, and then grab everything that isn't bolted down. I have millions of dollars in merchandise if I have a decent trade, and need to then send some off, assign consignment costs, etc.
I always saw the guideline as just that. If you are willing to wait a season, hire a seller, bring my items to a large trademeet, getting people interested. A +3 Bane (Evil Outsider) sword may have its hilt reworked by the Priests of Whacabalor for their Guardian of the Eastern Reach. They may even pay 80% of the original cost... But you are currently 1000 miles away. Do you want to pay to Teleport over to them, do all the Diplomacy and Knowledge (Religion) checks, and then travel with your loot?
Didn't think so. But that merchant will. That merchant makes his money on it. He probably is part of a trade union that has contacts all over the world. You're paying for their agency, and they say so. Now would a merchant possibly give an offer for well-traded items? Yeah, if you work with him and offer him first-pick. He may even offer you some work on the side... But since most people treat the Magic Shop as a 7/11 with
portable holes they don't do this.
Simple System (add the following percentage of your overall sale to your overall cost, Except for Pawn). :
10-15% - Pawn. You came up short on your Mage dues. You have a season to pay back a small overage of the overall loan. Not usual for adventurers, but it does allow you to save yourself from selling that
Holy Avenger just yet and gives you quick cash now.
+11-20% - Take this off your hands. You need quick cash for a Raise, the merchant will give it to you almost immediately (if he has cash on hand). This would be your standard deal, willy-nilly wham-bam. If the merchant is not himself a Caster look to pay for the Identify because he's been burned by adventurers before. The merchant may take 1d4+2 hours to gather the cash, plus the time for Identify but you'll have it by end of day. A merchant should be able to get 1-2 apprentices from the local Mage guild for assistance, and will tack that cost onto his usual fee at a discounted rate to you (let us say half of the ID cost if you agree to sale).
+21-30% - Finding a buyer (local). You pay the merchant his fees for this up front (let's say a 50GP per 2000 GP item worth). The merchant will look for a buyer using that extra up-front cash to hire runners, bribe the household staff of the noble lord to present him with their information, etc. If he has Gather Information and is above 3rd level (or takes a feat to boost it) he'll take 10, find a buyer, do all the inside work for you, and then make the sale. Buyers will require that they get their own Identify cast or, if a good work relationship exists, they will take the merchant's word. Allow around 1d4 days for the most expensive item, and then +1 for every 5000 GP of sales value you're discussing.
+31-60% - Finding a buyer (national). You pay the merchant traveling fees and up-front costs for all items. The merchant will want 150/2000 GP for all items sold this way. This is going to take awhile as the merchant will go atraveling, using this cash to pay for the costs of a band of low-level adventurers, lodging, and appropriate components for wards and protection. All items will also require Identify to be paid for, and for really high-end items you're looking at Identify and Legend Lore alongside to confirm that this
Holy Avenger wasn't stolen from the High Priest of Whacabalor's Eastern Champion

.
You're looking at a large overhead up front, but you get more money on the backend (45-55% overall? Nice). Now this will take longer by far (1d4 Months, +1 day per 5000GP over the first item). You can reduce this by half through providing any provisioning assistance you can (Teleport back AND forth, a borrowed
Bag of Holding, escorts provided from your personal retinue by Leadership, using social connections to get the group attached to a larger caravan, or an assistant to help with the bookkeeping and sale. You may also speed it up by hiring spellcasters or sellswords, but this will of course cut into your bottom line. For every benefit you give the merchant over the first you increase the return by 2% up to a maximum of 60%.
Example: The players just defeated the dreaded barbarian warrior Gimmick Bravehands. They have no need for two items, a
Lion's Shield and Breastplate of Command.
Item's overall cost (combined): 34570
Normal Sale's Price: 17285
The merchant, a renowned leader in the Scrivener's Guild who practices money-changing, assurance, and sale of high-end items to various noble houses in the area, has bought items at normal rate from you and likes what he sees. offers the following:
1 Day: 18773 (19015 at +10% base, 240 GP for the identify, wait until nightfall, he keeps the extra 5 silver).
5 Days: 20520 (21260 at +23% base, 240 for the Identify, 500 GP for greasing palms).
3 Months, 5 days: 23214 (25,754 at +49% base, 1800 GP for overhead, 500 for 2 Legend's Lore, 240GP for ID, he keeps the couple of silvers and coppers). The merchant must travel to a large Tradesmeet a couple thousand miles away next season, gather together his crew, etc.
The party's mage offers a Teleport to and fro for the merchant and his items so the Merchant hires two strong oxen and takes his trade goods along. The Mage is also able to cast Identify and Legend Lore and provides his services with confirmation from the Mage.
The time is cut down by the original four months and ten days, and the party Rogue [a senior member of the Brotherhood of Shadows] provides the merchant with a symbol to draw above his tents to prevent mercantile theft. For his thanks, the Merchant waives the Identify fees and provides an additional 5% of the profit.
1 month, 20 days: 24820 (26620, - 1800 GP overhead).
Can your party wait a whole 50 days for that cash? If they can they'll make 7k more than what they would have. If they wish to accompany to the location? Heck, there's the overhead cost in pocket and the chance for more adventures

.
Slainte,
-Loonook.