There are rules for the cost of magic items (and those cheap common ones I mentioned should be just 50 gp so run out now and get a Talking Doll

). And Xanathars has a whole page devoted to buying magic items and another page for selling magic items and nearly two pages for crafting magic items. But yeah, RAW you can't buy a magic item? RAW? Sorry, RAW magic items can be bought.
But also note: I didn't say anything about walmart in my post. I said, with the right contacts, you could find a "store" in WATERDEEP. You can BUY or SELL anything in Waterdeep. Why did you even mention walmart? See, a better argument would have been Pit Fighting, also in Xanathar's. Am I forced to introduce pit fighting into my world just because the book has rules for doing so? No, of course not, and you might have suggested that buying magic items doesn't happen in the One Shot compaign world and the group could discuss that. But RAW?
Finally, the designers. I even said in my post "Maybe RAI you cannot buy magic items." But just because the sacred numbers don't account for magic items, doesn't mean they can or cannot be purchased.
Bringing this back to the thread at hand:
Since there is no actual world outside the One Shots, I would think it would simpler to assume magic items are Commissioned. You state to the DM, in the recruiting thread of the next game, that prior to the adventure hook, you wanted to commission a Moon Touched Sword. If the DM approves, you arrived in the city where the adventure starts with the item (or are picking up the item there).
That's my suggestion. (And I still need to actually make some characters as I need to be in more games.)