How do most people pay for items at the Exhibitor hall? Do they have debit/credit machines or is it just cash? Do they give receipts? 

Most have credit card machines (gamers will spend a lot of money).  The cheaper the items at the booth, the less likely to have a machine.

Almost all vendors give reciepts normally (and all will give you one if you ask).  Make sure you get reciepts if you are travelling internationally.

Cash is king, but don't expect any discounts or deals for using it or haggling.  Since many of the exhibitors are the games manufacturers/distributors themselves, you often can contact them afterwards for warranty issues. 

My plan is to carry around $600USD cash with me for the hall (not all at once or in one place though), and card any big items (Big meaning $100+ price)

The one thing to watch out for if you're using a credit card is that as you rumble through the dealers' hall (ideally!) in a mad shopping frenzy your card's provider will see charges rolling in from all over the USA all at once, based on where the company you're buying from is based, and this might act as a red flag and get your card shut down.  Best to call your credit card company before you head for Indy and tell them to expect this, and over what period.

Not to mention the ever present possibility of people lurking around with credit card skimmers in a crowd of 60,000+ people with enough disposable income to travel to spend four+ days playing games. Cash is pretty safe, all things considered :)

If you have a Chip card you want to use, know its PIN.

Between the wide variety of charging systems in use by everyone around the convention (not just the vendors), the ongoing slow conversion to Chip card systems here in the US, and the occasional "perfectly working Internet" in the ICC, you'll see practically every type of authentication in use. (Imprinter, Swipe, Chip, Swipe and Signature, Chip and Signature, Chip and PIN...)  If a vendor's system chooses Chip and PIN, you might be out of luck if you don't know your PIN.  (Also another reason you might prefer cash. :)

Quite a few vendors use square to get around the exorbitant wifi/internet fees the convention hall charges so be aware if your uncomfortable having someone scan your card with thier phone.

@lanefan: My credit card company didn't say a word about the charges but I freaked out for a minute when I got my bill and saw charges from half a dozen places across the country, my first thought was identity theft till I calmed down and saw the company names so yes that is a well advised warning.

For several years in a row, my card would get shut down on day 1 of gencon and I would have to call the number on the back of the card to let them know that I am at a convention making purchases from vendors whose accounts are based all over the country, as well as brick and mortar locations in indy.  The last year or two it hasn't happened. The fraud detection algorithms have gotten really good. The system probably knows that the vendor is using a portable device, and where the device was at the time of the charge. The system probably can identify that the unusual activity is linked to gencon, even if a human operator looking into the activity would have to google gencon.

  I get shut down when I travel for work less often these days as well. But when someone did get my card numbers a few months ago, the system knew about that very quickly and accurately too.

In the past Wizkids has been cash only.  

The nice customer service person at Ticketmaster also then gave me that person's email address they used to place the order when I went to try to track them down :)