[RPG] A player asked me for one specific magic item, should I “sell it” to him before the game

To quote the creation rules you cited:

To create magic items, spellcasters use special feats which allow them to invest time and money in an item's creation. At the end of this process, the spellcaster must make a single skill check (usually Spellcraft, but sometimes another skill) to finish the item. [...] It is possible for more than one character to cooperate in the creation of an item, with each participant providing one or more of the prerequisites.

The rules here clearly state that yes, the necromancer could fulfill the requirement needed for Necromancer's Athame if the necromancer had Craft Wondrous Item. What they are less clear on is exactly how this would work.

Helping someone work on a magic item would be an "Aid Another" action to influence the outcome of this final skill roll.

Here are the complete Aid Another rules:

You can help someone achieve success on a skill check by making the same kind of skill check in a cooperative effort. If you roll a 10 or higher on your check, the character you're helping gets a +2 bonus on his or her check. (You can't take 10 on a skill check to aid another.) In many cases, a character's help won't be beneficial, or only a limited number of characters can help at once.

In cases where the skill restricts who can achieve certain results, such as trying to open a lock using Disable Device, you can't aid another to grant a bonus to a task that your character couldn't achieve alone. The GM might impose further restrictions to aiding another on a case-by-case basis as well.

First, notice that you are required to make "the same kind of skill check in a cooperative effort." As a GM, I would rule that this means that all participants (the creator and the aiders) must work on the item 8 hours a day (or however long the creator is working) throughout its entirely. The aiders cannot simply show up at the end of the creation and aid for the final check.

Second, since a "cooperative effort" must be made, I would rule that the creator must expend as much effort as working unaided. This means that paralyzed characters cannot craft magic items, unless they themselves have some other way to work with the necessary tools.

Third, rules-as-written states "you can't aid another to grant a bonus to a task that your character couldn't achieve alone." This would mean that every character involved, be they creator or aider, must be technically able to craft the magic item themselves. However, note the following lines from the creation rules:

The DC to create a magic item increases by 5 for each prerequisite the caster does not meet. The only exception to this is the requisite item creation feat, which is mandatory.

Taking this into consideration, every character who can make a spellcraft roll and who has the requisite item creation feat can aid in the creation of a magic item. Each aider would fulfill any requirement they could, and add +2 to the creator's skill check if they succeeded in their roll.

For each prerequisite not met by the creator or any aider, increase the DC by 5, then roll the creator's skill check, adding to it for each successful aider.

To address your final question about the number of people working on a magic item, the rules for Aid Another leave this up to the GM. I would argue that the number of people lending aid is proportional to the complexity of the item. A scroll, ioun stone, or wand, for example, may not have any room for aiders; they would just get in the way. An Apparatus of the Crab, on the other hand, may have a dozen people working on its many parts.

I think it's just to ensure that PCs can only craft items during downtime.

It forces you to take a full day out to craft small items. The one day minimum helps prevent weird cases like:

• Three scrolls taking the same time to scribe as one scroll
• Crafting twenty feather tokens in one day
• Scribing an unused spell into a scroll before you go to sleep, just to avoid wasting it
• Crafting an item casually in an hour, without taking a full day of downtime

Other than that, there are no major balance issues. The amount of downtime isn't specified in the rules so crafting isn't strongly balanced against it as a limited resource. You're still spending your own gold to craft the items. I think your house rule would be reasonable.

Remember also that you're already limited by the number of spells you can cast per day: you have to cast each item's prerequisite spell once each day while crafting. To craft five feather tokens, you'd have to prepare major creation five times (or hire someone else who can, or cast from scrolls, or raise the item creation DC).

Edit: Another reason is to simplify item creation time down to increments of whole days.

Originally in D&D 3.5, you couldn't craft items while adventuring and even small items took a full 8 hour day. Pathfinder eased those requirements, but kept the 1/day limit. One benefit is that it divides item creation into whole days, which is simpler than tracking individual hours. It's not overpowered, just a restriction Pathfinder inherited from D&D 3.5.

You can remove the 1/day limit if you're willing to handle the extra complexity.