The question ran thus:
"My group found a small horde of coins that contained some scrolls and they immediately wanted to sell one of the higher-level scrolls for some much needed adventuring capital. I'm curious ... When the party does come across temporary magic like scrolls and potions, wands and such, how do you handle the sale of such items ..."
The short answer is that I don't sell, and I don't have vendors buy, magical items. I could imagine a party giving an item to someone in order to receive goodwill, but selling it? No, I can't see that.
It is quite possible that I have this position because in my experience, parties are not stupid enough to rid themselves of an item that may be, at some future point, a matter of life and death to them. It has happened more times than I can describe that some obscure scroll that appeared to have no immediate application turned out to be crucial ... and it is usually something I've forgotten that I gave the party a year ago, and something that the player is triumphant about declaring that they have.
I don't know what the scroll is that they were trying to sell. From the context, there was more than one scroll located and that may have something to do with the party's decision: too much magic. I tend not to throw out very many pieces myself, and the party can run four or five sessions without seeing so much as a +1 dagger. As such, magic is hotly contested for when it appears, and is carefully parsed out among the existing party members. Where it comes to scrolls or potions, my spellcasters would be awfully pissed that this power wasn't immediately put into their possession, and to hell with the fighters and thieves bent on selling it - spellcasters will take any additional power they can get, especially if it means striking an enemy from a distance.
And when a piece of magic is lost, as sometimes happens, it remains a sore spot in a player's memory for a long time.
As well, even if something turns up that no one can rationally use - a +1 battle axe turned up at the end of the massive battle and no one was proficient - the party will set it aside for the arrival of a new henchman, who will then be granted the weapon. Thus every random item is carefully sequestered with some individual, who then cherishes it as the only magic item they have, which is commonly the case. Generally, the distribution of magic to player is about 1 item for every 2 levels. You don't sell something that rare.
So getting past the issue with even the earliest modules (such as the Keep on the Borderlands) having three magic items in every damn room, what other issues do I have?
Well, there is the question, why would someone in a town buy a magic item?
Listen, I can't speak for the gentle reader's world, but mine is full of charlatans, ready to sell you a magic potion on any given moment that promises to cure the warts on your feet or make you BFF with Zeus. As such, no one with half a brain believes it when four rough and tumble dust-covered strangers blow into town claim that the bottle in their hands will, without a doubt, grant them the power of clairaudience.
But of course, there's detect magic and identify, so if we can find a legitimate mage in the town, who possesses those spells, then we can at least prove that our claims are valid. Which is nice and all, but still doesn't explain why I, the apothecary, have any interest in choking up 900 g.p. to pay for this bottle. Look, I just don't have customers dropping in here every day that make it worthwhile for me to keep something like this in stock ... it's like asking the place to get robbed the first time I demonstrate the item to a buyer. I don't have any personal use for the item, since I don't adventure and if I did need to do the clairaudience trip on some other fellow, I'd write my contacts who have that spell to drop by for an afternoon and give it a go. Not only that, but it's not the sort of thing you want lying around when the town guard chooses to roust the joint. Besides, I have rent to pay, don't I, and practical materials that need buying and I think my coin's better placed in that direction. So yes, that's a fine potion there, but I think I'll give it a pass.
I think its imagined that the town magic dealer hasn't anything better to do than to keep tens of thousands of gold pieces in stock, just waiting for the next group of adventurers to hit town ... and that somehow this reality has escaped the attentions of the thieves' guild, the assassin's guild and half the underground villians grouped together in god knows what sewer. I think the imagined idea is that the dealer is 9th level and no one would mess with him - except that my party has killed a 9th level mage (surrounded by about 400 followers) and there just isn't anyone, anywhere, who's that invulnerable. The turnover rate on a shop's storage just isn't that high that it's worth blowing money on materials and effort to make magic, or coin to buy magic ... not if you want to run a world that runs according to reason.
Of course, most DMs play the magic shop with the sort of rationale that was invented in games like Ultima twenty-five years ago, and everything in the above paragraph sounds like me saying "blah, blah-blah blah blah" for 151 words. But I'm the sort of fellow watching the movie asking, "Wouldn't it make sense to kill all the ewoks with napalm before starting construction?"
I want things to make sense.
So my advice would be, give more coin and give less magic. They won't need the coin, so they won't need to sell the magic, and the issue will cease to occur.