This is the most often misunderstood aspect of the gun laws in Ill mostly spread by FFL dealers as it IS a requirement for FFL dealers--
There is NO waiting period for legal PRIVATE sale--
Yes, it is one of the most misunderstood aspects of Illinois law. However, I must respectfully disagree. I am not a lawyer (just a lowly pre-law student), but I have noticed many discrepencies between what Illinois residents (including state and local police agencies) believe the law is, and what the law actually states. I would strongly suggest you read over Illinois Compiled Statutes, Chapter 720, Act 5, Article 24 ("Deadly Weapons") in its entirety to find many of the discrepencies.
Regarding "waiting period" laws, Illinois statutes are very clear. Section 24-3 of the aforementioned Article, titled "Unlawful Sale of Firearms," states in subsection (A) paragraph (g):
Delivers any firearm of a size which may be concealed upon
the person, incidental to a sale, without withholding delivery of
such firearm for at least 72 hours after application for its
purchase has been made, or delivers any rifle, shotgun or other long
gun, incidental to a sale, without withholding delivery of such
rifle, shotgun or other long gun for at least 24 hours after
application for its purchase has been made. However, this paragraph
(g) does not apply to: (1) the sale of a firearm to a law
enforcement officer or a person who desires to purchase a firearm
for use in promoting the public interest incident to his or her
employment as a bank guard, armed truck guard, or other similar
employment; (2) a mail order sale of a firearm to a nonresident of
Illinois under which the firearm is mailed to a point outside the
boundaries of Illinois; (3) the sale of a firearm to a nonresident
of Illinois while at a firearm showing or display recognized by the
Illinois Department of State Police; or (4) the sale of a firearm to
a dealer licensed under the Federal Firearms Act of the United
States.
Under this statute, it is established that it shall be unlawful to deliver without withholding for the required period of time, unless one of the exemptions listed is true. Please note that the exemptions are based on the occupational or locational status of the
buyer, not the seller, so your status as a private citizen does not give you any exemption. Also, exemptions (2) and (3) should not be taken as a green light to sell firearms to non-residents (this right is reserved for FFL only due to current legislation in Federal GCA '68 and its amendments).
In short, regardless of the seller's status as a private citizen and not an FFL, a sale of a firearm
must be withheld for a period of 72 hours or 24 hours, depending on the type of firearm, unless the buyer is in law enforcement, armed security, or an FFL. I would urge you to be very careful in how you approach any firearm sale in the state of Illinois and ask any prospective buyer to make an application for purchase at least 24 (or 72 hours for a handgun) in advance of the time for delivery. Record the time and date the buyer asked to purchase the firearm, and refuse any delivery prior to the required minimum withholding period. Also record the date and time you delivered the firearm to them, and have them initial or sign the paperwork. Call me paranoid, but I even prefer to have someone there to witness the transaction and delivery.
Keep in mind that in the same section, subsection (C)(1), the penalty for violating subsection (A)(g) is established as a Class IV Felony, which carries a maximum sentence of 3 years in state prison, and a fine of up to $25,000. You would also lose your right to possess any firearm under Federal and Illinois law.
To my understanding, this issue has not been addressed properly and clarified until perhaps within the last couple months when Illinois State Police posted the answer to the waiting period question in their online FAQ at:
http://www.isp.state.il.us/
If ever called on it, your best bet is probably to make an affirmative defense based on ignorance as set out in ILCS 5/4-8.