You're allowed to bring anything that is for the trade show that you can't sell or can't use to generate profit. You can generate leads by having meetings with people (therefore you can bring a computer, business cards, etc.) but you can't do "work".
You're technically not allowed to sell merchandise within the U.S. if you're not an American corporation with a registered tax number in the state where you'll be showing. This means that pretty much every booth you see at PAX which sells stuff (i.e. all of them) is doing so illegally. Usually no one cares, though there have been rumors of IRS folks walking around and fining people at shows (never seen it though throughout 3 PAXes).
You're allowed to give stuff away as promotion, but at the border they'll be the judge of whether it's promotional or for sale (spoiler alert: they like having power and like abusing it even more, don't take too many chances).
My friend once got denied entry to the US because he had a camera with him to shoot some footage of the show... they decided he was "going to do work" and therefore wasn't allowed. So if anyone asks, you can be doing business development but I believe as soon as you're signing a contract it counts as work and is thus not legal.
You definitely can't bring physical copies of your game to sell, and you're not technically allowed to sell digital copies either (if you're receiving cash for them on-site). I'm not sure how anyone could know whether or not that was happening, but I'm just talking in terms of legality.
Here's some documentation taken from the CBP.gov site. You'll need:
The CBP 7523 should only be filled if you're bringing in merchandise, though the limit of what you're legally allowed to take seems to be extremely subjective. Last time, we brought pins that we were giving away (1000 pins that were valued at $0.25 each) and we had to pay about $10 in duties for them.
The following bit is very important: if what you're bringing has a value of over $2500 USD or if you're shipping supplies, the best option is to get a Temporary Importation Under Bond (also known as a Carnet). Some info about what a Carnet is can be found here, and how to apply for one can be found here, along with the processing fees involved. This involves going to the Chamber of Commerce and registering, and it also involves paying 40% of the value of the goods you're bringing across, which gets refunded to you when you come back (though I've heard mysterious stories about it not being fully refunded).
I believe this information was from a call with a border agent, where I was also told to bring:
I'm not actually sure if the 2877 is needed if the FCC form is already being filled out, but we're doing both just in case.
Note that this is the best way to go through without trouble, and have all the documentation ready in case they decide to be assholes. We've had really bad experiences with the CBP, and I imagine we'll continue to have issues forever, so we really make sure that everything is legit and well documented. Most people get through without any searches or without any documentation, but it's a risk you've got to be willing to take. We've never had issues where we were denied entry or had to pay fees, but we have been delayed by a few hours with some paperwork.
Anyway, this was our experience and hopefully we can help someone avoid issues in the future or clarify questions that they've had.
Rich and the Clever Endeavour Team :)