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.

Generally, no...

The Adventurers League Players Pack contains the documents that detail your options in AL play. Purchases are restricted to items in the PHB. The only potion available for sale without special documentation is the basic Potion of Healing. There are no other magic items in the PHB - no potions, no scrolls - so they can't be purchased either.

...unless you're of sufficient rank in a faction...

Characters who reach rank 3 in their faction can use downtime and gold to acquire magic items from a very limited list: +1 weapons, shields, and armor are available to all faction members, and each faction allows access to a single uncommon item and a single rare item.

...attend an event featuring Fai Chen's Fantastic Faire...

Fai Chen's is available to public events of sufficient size that register with WoTC for it in advance. Fai Chen trades (but does not sell) permanent magic items, sells scrolls up to 5th level (one per transaction per event), strange trinkets, and occasionally special mounts and property deeds.

...or adventure in Chult.

There is a very small number of magical weapons and shields, plus scrolls and potions, available for purchase only to characters playing Season 7 content - DDAL modules and Tomb of Annihilation. These purchases are only available as a downtime activity.