It's not that the "kick in the door" style requires magic item purchasing, it's that purchasing magic items is a common option in 1E, 3E, and 4E (discouraged in 2E) -snip-
<emphasis mine>
You've mentioned this repeatedly and I simply have to disagree. Magic item "purchasing" was most certainly NOT a "common option" in 1e as in 3 or 4e.
I can not think of a single 1e game I was EVER in or ran or heard of wherein the above emphasized statement was true. Other than potions and scrolls..maybe wands or a +1 weapon/armor.
Yes, the 1e DMG has "GP Sale Value". That is, if you had the magic item and wanted to get rid of it for the cash-money instead. Not that there were NPCs hanging around just waiting for you to drop down 25,000 gp for your Bag of Holding.
I personally never saw play bitd in which players received high value many magic items that they then turned around and "sold off." If you got good stuff. You KEPT it. Even if you didn't use it (cuz you might need it some day). And, for sure, no one/game I ever saw or heard of involved "magic item shopping" [beyond, as I said, simple/small consumable items].
OR, I do recall, selling items off for the gold to get more XP. Since gp was worth XP and the GP values almost always exceed the straight XP values -1e gave separate XP value on the items-, folks could do this to "work the system."...and a handy way to raise large funds when/if you DID want to build that stronghold or raise/pay an army or craft spells or items of your own.
Could you take that value from the 1e DMG and say, "C'mon down to Jack's Magic & Might Marketorium for all your magic-Magic-MAGIC Item need <MAGIC ITEMS!> at low-Low-LOW Gold pieces! ANY magic items you want! <ANY ITEMS!> At DMG prices! <MAGIC ITEMS AT DMG PRICES!> That's Jack's Magic & Might Marketorium. Sun up to Sun down. Everyday except Harvest. Exit 42 off King's Toll Road to Ogre Bridge Blvd. Remember, if it's not M&M, it ain't worth Jack."
Sure. You could. But that is most certainly NOT 1e's RAI or W. No one I ever played with saw them as catalogue entries they could go swing by somewhere and pick it up.
So continuing to site how this was so common and intended in 1e -as it was in 3 and 4e- is kind of...neither true nor fact.