They are from people crafting them for the sole purpose of selling them. One of the good things about buying a recipe is that it is not only cheaper than buying the item itself, but you can then use the recipe to put together those raw materials again to make an item to sell, hopefully for more than you would've gotten for just the raw materials. Note that crafted items will sometimes get special attributes that you can't get on dropped/store-bought items, making them even more valuable.
While, generally speaking, you are limited to ten floors of a dungeon per day, each dungeon is split into 3 separate Tiers. Each of these tiers starts at a different depth - 0, 8, and 18, So if you have Tier 2 Clearance, you can start at floor 8, and go all the way to floor 20 (no energy costs to access floors 13 and 18).
Likewise, if you start at floor 18 (tier 3 clearance), you can make it all the way to the core, Level 29, and still have 10 energy left over.
The one downside of this is that starting at Tier 2 or 3 costs Crowns (the in-game currency) -- 200 or 500 respectively. However, if you work your way through one gate sequentially - getting to floor 8 the first day, getting to floor 18 on the next, etc. you will avoid having to pay the cost in crowns.
Since gates have an 8-day cycle, this means you will be able to delve all the way to the Core 6 out of 8 days, avoiding the 'tax' for jumping directly into the deeper tiers. Alternatively, just jump directly into Tier 2 and Tier 3 without clearance; the Crown tax isn't that bad, and you will generally recoup your entry fee within the first floor or two.
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The only unique property of the alchemer is their charge attack, a bullet that shatters into smaller bullets. But on a more general note, make sure you see the 5* versions of the guns in action before making comparisons - Alchemer bullets aren't slow; when upgraded, they're one of the fastest traveling bullets in the game!
This is most obvious at the 5* versions, where a well placed alchemer charge shot can shatter within the body of a larger enemy, causing all the resultant bullets to instantly hit the same monster. This means that the alchemer possesses some of the best burst DPS available, though it may be hard to pull this off consistently.
The Shadow Driver series was also, until recently, the best pure shadow gun (Biohazard has slower bullets and a much less useful charge attack).
Finally, the reload time is constant across guns - the difference is the clip size. This can be worked around, however - just pause between each shot, and you'll never have to reload.