I've checked the FAQs and policies, and I can't find any rule either allowing or preventing me from selling a book/game/whatever to another attendee.  This is a sale that would be pre-arranged ahead of time; I'm not going to be standing in the concourse hawking my stuff.

The Event Host Policy prohibits it for EOs, but this is just me, selling stuff I no longer need to someone else, and not doing it through the auction.

If there is a set policy on this, where do I find it?

From the Exhibitors rules and regulations:  Exhibiting Rules and Regulations

General Rules and Regulations

1. Handbill distribution is permitted only within the exhibiting company’s booth or at exhibitor sponsored events. No material may be posted on convention walls, fixtures, or furniture. Violators are responsible for all fees and damages incurred from the removal of such postings.
   
2. Adhesive stickers are prohibited in the Convention Center. Exhibitors are liable for all costs and expenses associated with sticker removal.
   
3. Absolutely no selling, soliciting, or handbill distribution is allowed outside of rented booth spaces, unless such a distribution has been secured through an official Gen Con Event Marketing Program or Sponsorship. Violators will be removed from the convention. In the case of violating exhibitors, the exhibitor and all employees will be removed from the convention.
   
4. Exhibitors must comply with, and be bound by, all laws, ordinances, and regulations of the country, state, county, city, facility, and, whenever applicable, police and fire departments.
   
5. Authorized company representatives must staff exhibits during all exhibit hall hours.
   
6. No exhibitor or member of an exhibit may promote, display, or behave in a manner considered offensive to decency or good taste as determined by Show Management.
   

That being said, this would only apply to the convention property.  What you do outside of the convention (ICC / Lucas Oil / Food truck-Beer Tent) is your business.  Of course if you are selling that much, you may have to worry about running a business without a license.  

That's under the exhibitor rules, it doesn't say anything about attendees.

And I'm talking about maybe 5 books.

Assume this is something like you were on BGG or a forum and you setup a sale of an item.  If that worried about breaking any rules, can just step outside the building and complete the transaction.  Plenty of people on game forums that are setting up sales/transactions at Gencon (and other cons).   As long as you aren't trying to solicit sales (like holding up a sign that says '5 books for sale!') no one is going to pay that much attention to someone handing over an item and being paid.

That’s what I figured, but it would be nice to get an official confirmation from Gen Con.  All the prohibitions against selling items are contained in documents the basic attendee doesn’t read (Exhibitor Rules & Event Host Policy).

Mike Boozer
Customer Service & Event Team Manager
Gen Con LLC

Thanks Mike, I'll take any sales off-site.

ETA: Might I suggest that you add the sales policies to one of the Attendee FAQs?  This question pops up now and then on the Friends of GC FB page, and while everyone “knows” it violates the rules, there’s nothing in the Attendee FAQs to point to.

Yikes, I definitely had someone offer to buy one of my older Gen-Con Auction pins off me last year; it would never have even occurred to me that it was literally against the rules to do that.  Good thing I only wear things I consider priceless and would never sell anyway ;)