TL;DR: Spell scroll is a consumable item. It holds a spell which can be cast from it or copied into a wizard's spellbook, both of which destroy the scroll. Spell on a scroll refers to that specific spell, which is incidentally written on a scroll.
Spell scrolls (as you'd find in the treasure tables) are spells already prepared onto the scroll and contain some/all of the magic needed to cast them within them (which is why creating them is more than just copying things out of a book).
The second passage you are quoting describes how wizards copy any spell that they find written on a piece of parchment, in a book or on the back of a box of your favourite Orcish breakfast cereal, and put it into their spellbook.
Copying a Spell into the Book. When you find a wizard spell of 1st level or highter, you can add it to your spellbook if its of a level for which you have spell slots and if you can spare the time to decipher and copy it.
Copying a spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation.
The rules in the DMG are specifically about spells on spell scrolls (the type found in the random treasure tables etc.)
In addition to the rules on copying any spell into your spellbook you also have to follow these rules specific to spell scrolls.
A wizard spell on a spell scroll can be copied just as spells in a spellbook can be copied. When a spell is copied from a spell scroll, the copier must succeed on an Intelligence(Arcana) check with a DC equal to 10 + the spell's level. If the check succeeds, the spell is successfully copied. Whether the check succeeds or fails, the spell scroll is destroyed.
Basically you get one shot to understand enough about this spell scroll before the magic is used up and you can't copy it into your book.
So yes, there's a difference between a plain written spell on any old piece of parchment, and a spell scroll.
Additionally: As KorvinStarmast brought up in the comments you could have someone else help you with this check using the Help action.
Help
You can lend aid to another creature in the completion of a task. When you take the Help action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn.
Bare in mind that many DMs rule that you can only help with tasks that it makes sense for you to be able to aid someone with. In this instance I would think that at the very least they'd need to have the spell in question on their own class's spell list, or perhaps even be a wizard themselves.
Your goal seems to be to have the NPC profit every time it is cast. It looks like you value spells without material components, but why not embrace (expensive, consumed on use) components?
Instead of making and selling scrolls, the easiest way to charge someone every time they cast the spell is to build the spell with an "expensive" material component you have monopoly over. This is something the Wizard can do himself at any level, and will fall within/sidestep the legality concerns by having built in rights management (punishment is irrelevant if it simply doesn't work without the requisite consumed-on-use license token).
Best Answer
The Player's Handbook on Spells Copied from Another’s Spellbook or a Scroll says
(Link mine.) So, by the core rules, a wizard can copy any written spell into her spellbook.
However, the Rule's Compendium says, "Spellcasters who use spellbooks can add a spell to their book whenever they find one on a scroll or in another caster’s spellbook. The spell to be copied must be on the copier’s class spell list" (160), and, here, the Rules Compendium agrees with one of the game's designers (see this answer). Ask your DM. This DM has found the game easier to manage for both the DM and the wizard if everyone agrees to limit wizards to scribing into spellbooks only spells that are on the sorcerer/wizard spells list, but another DM's experience may be different.
A typical single-classed wizard can't cast spells that don't appear on the sorcerer/wizard spell list even if they are in the wizard's spellbook
The presence of a spell in a wizard's spellbook doesn't enable that wizard to cast that spell.
The Player's Handbook in the description of the wizard class feature spells says, "A wizard casts arcane spells (the same type of spells available to sorcerers and bards), which are drawn from the sorcerer/wizard spell list…" (56). For example, the typical single-classed wizard just can't cast a spell that's exclusive to the cleric spell list even if the wizard understands that cleric-spell-list-only spell and has it in her spellbook.
In short, a spell must be on the sorcerer/wizard spell list for a wizard to cast it. Alternatively, an individual wizard must uniquely add the spell to her own wizard spell list.
(For example, this DM has always assumed that a creature adds the new spells to the creature's appropriate spell list when the creature succeeds at Researching Original Spells (DMG 198)—as always, bear in mind that researching original spells is vaguely defined, easily abused, and almost entirely at the DM's discretion.)