I think there's two reasons why it would make sense for magic item creation costs to be as high or higher than purchase cost, that fit with the idea of D&D and the stories portrayed:

Most of the permanent stuff for sale is ancient

For most of it, it doesn't matter how much gp a caster once spent for it. The magic item for sale in the Bazaar of the Bizarre isn't made by the owner, or anyone he knows. It's made by an ancient wizard, who died centuries ago, and isn't going to see a penny of that money. It was probably looted by adventurers, who then decided they didn't need it all that much, so they sold it for whatever they could get for it and now it's for sale for whatever the item's owner thinks he can get for it.

The reason he asks you for 2000gp, even though you can make it for 2000gp is because A) most of the people who can afford a magic item, could also make one themselves and B) he's now undercutting any bored Wizard who tries to compete with freshly made magic gear. Considering these magic items are practically indestructible and require next to no maintenance, there's eons of time that they've been made in and most are simply still around to be found.

The market for permanent magic items is terrible because the stuff literally lasts forever and there's a whole class of people whose only job is to venture into the wilds, "liberate" the items and then sell them cheap so they can get some more potions for their next "adventure". Sellers are simply responding to this dynamic.

Most of the consumable stuff is made by specialists

The reason your Wizard takes 4 days to write a scroll and spends a 100gp on it is, for a major part, because your Wizard is a firebreathing, lightning throwing, people charming, monster summoning murder-machine and not a scribe.

Probably if you decided to spend 90% of your time learning how to write a Scroll of Burning Hands faster and cheaper than usual, you could also learn to do it in half the time and for half the cost and earn some money selling those scrolls to the other Wizards who don't perfect the art of calligraphy but instead waste their time going out to kill things and take their stuff.

You simply cannot reasonably compete with the people who dedicate their lives to creating consumable magic items and never learn to survive adventures. (And the reason there aren't any rules for doing so is because this is Dungeons & Dragons and any such character would be an NPC, not a player character)

Why you can't make money selling magic items

Ultimately, what it comes down to is this: making a lot of money off of creating and selling magic is boring and not what D&D is about, so the standard rules don't allow for it. The above is just flavoring for why it's like this.

Any experienced DM who can turn "making and selling +1 swords" into a fun play session will have enough experience to tweak (or disregard) the rules so that it works.

Any DM who doesn´t have that level of experience cannot accidentally screw up his game by showing his players how to make loads of money without actually risking their hides in the adventures that the game is about.

It's a win for everyone.

Creating custom items is always a matter of negotiation and judgment; there are no hard and fast rules. The books provide some guidelines, but are always quick to point out that they cannot be relied upon, and that the DM is always going to have to judge suggested custom items on a case-by-case basis.

The first guideline is to compare your desired item against the closest available comparable items, and adjust from there. Thus, to allow a ring, such as Devlin’s, to do what the quiver of Anariel does, you would start with the quiver of Anariel as your basis for the cost of doing so.

Furthermore, the passage on page 233 of Magic Item Compendium is really talking about adding new magical abilities onto an item that could otherwise have that ability. But ultimately, you can add anything anywhere as long as the DM allows it. The aforementioned guidelines suggest a 50% surcharge to add something to an unusual slot. So to put the quiver of Anariel effect on a ring should, by these guidelines, cost 50% more. On the other hand, the guidelines also suggest that something that does not use one of the usual magic item slots should cost double, so a magic quiver (which doesn’t prevent the use of other items) presumably costs double. So the guidelines would say that the ring should cost approximately 75% of what the corresponding quiver costs.

However, the problem here is that the quiver of Anariel is ludicrously overpriced, particularly at the regular arrows level. My games routinely ignore tracking mundane arrows in general, so they are literally charging 28,000 gp for a magic quiver that does what any quiver in my games does, by default.

On the other hand, magic arrows are a much bigger deal than mundane ones. Magic arrows partially stack with the magic properties on the bow shooting them, which is a big deal. While I’d say even Devlin’s ring is on the expensive side for mundane arrows, for magic arrows the cost definitely should be much higher.

So instead, I’m going to make a different suggestion: consider magic ammunition as if it were a 50-charge item, like wand. After all, enhancing 50 arrows costs the same as a sword. So if a 50-charge spell-effect item costs spell level × caster level × 750 gp, and a use-activated at-will item costs spell level × caster level × 2,000 gp, you’re looking at the use-activated version costing 2⅔× what the 50-charge version does. So if you apply a 2⅔× multiplier to the cost of 50 magic arrows (which is the same as the cost of a single magic sword), you have at least one reasonable idea for what this effect should cost.

The results are 5334 gp for +1, 21,334 gp for +2, 48,000 gp for +3, 85,334 gp for +4, and 133,334 gp for +5. Most likely you would ideally use equivalent levels of special weapon properties rather than straight enhancement bonuses, though.