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.

1.Can you use silver pieces for silver sling ammunition?

[When first writing this I'd not considered the mass of the coin compared to sling bullets. Both Dale M in his answer and Trish in her comment make good points that a GM might also... weigh when exercising their discretion.]

A. Yes, at your GM's discretion.

Improvised weapons are completely at the GM's discretion, per "Improvised Weapons" at PHB pp.147-148.

One can easily imagine a GM saying "it takes you a few minutes to mangle the silvers enough to fly as well as a stone, but that's not a problem." Or a GM saying "purchased sling bullets (PHB p.150) are akin to 18th-C. bullets: crafted materials that will do lethal harm at range. You can't just use any-old rock or coin." And there's nothing in the rules to tip the scales in one direction or the other.

B. Yes, with some downtime, and perhaps a little less discretion.

If you craft these silver bullets (PHB p.187), you can make a lot of silver sling bullets. Like, 2500 in a single day.¹ Again, crafting requires a lot of work with your GM, but I think there's a much better case to be made that even one untrained in weaponsmithing could mangle coins into pretty-good ammo-shape given a day and a hammer.

You could even call the first hundred your "practice bullets," then re-craft those raw materials, selecting the best twenty to put in your pouch, in under an hour!² All for just the cost of 10 sp.

[Note: there's nothing actually in the book about crafting over "partial" days. Union rules may require you to work the full day.]

2.What attack bonus/damage options would exist?

Whether or not this ammunition is similar "enough" to "real" sling bullets to allow you to exercise proficiency is, again, completely at your GM's discretion per "Improvised Weapons."

On the damage front, you've lucked out: improvised weapons do d4 damage unless it "is similar to an actual weapon and can be treated as such." But your sling bullets already do d4 damage, so either way the GM rules you're looking at d4 damage.³

Lastly, there's nothing in "Improvised Weapons" nor in Sharpshooter to rule out sharpshooting with these coin-bullets. Unless, as already discussed, the GM rules that you're not proficient in mangled-coins-in-twirled-fabric-straps.

^{1 - That's 5gp-worth of crafting divided by 4cp/20 bullets, for 2500 total bullets, given the input materials. Of course (assuming one coin per bullet), if you actually had that kind of silver at hand, you'd not need to ask this question. So you'll get to take a long lunch and clock out early, probably.}

^{Again, I had not originally considered mass; If you like Dale M's answer these numbers would need to be adjusted by a factor of ~4.}

^{2 - So that's 200 bullets crafted (albeit destroying 100 and rejecting another 80) for a total value-created of 40cp, or less than one-tenth a crafting day.}

^{3 - Unless the GM decides, as is completely their prerogative, that these improvised bullets should have a different damage. Like, say, 1 or 1d2.}