I'll add my 2 bits:
If you buy something, and *don't* have a trailer it will be a major hassle finding one, getting back to get your item, or paying someone more than $25 to to deliver it for you. If it were me I'd bite the bullet and bring the trailer.
Definitely bring the tools, you may want to remove the legs (recommended) of the pinball cabinet for shipping. Some pins require tools to lay down the back, some don't. You are very likely to find project pins at the arcade, going for $200 to $500 in various condition. Good condition pins will be pricey, $850 - $2000?
Set your price ahead of time, don't get caught up in bidding. There's always something at the next auction, unless your personal holy grail is there. In that case, you're on your own... Hence my $1150 Machine Bride of Pinbot

If it's an open trailer, you will want something to protect the machines. Tarp, rope (rope to hold the machine to the trailer regardless of the tarp). You often can buy plastic wrap at the auction to cucoon your new prize before shipping. It would stink to let it get road damaged on the trip home.
Get there early to allow yourself enough time to poke around. I'd get there at 8, 9 at the latest.
If you can bring wads of cash you'll get away cheaper. They give a 3% discount for cash (really they charge a 3% premium for credit cards, but can't say that). They don't take checks. Cash or credit card, or certified letter of credit from your bank apparently though I've never seen anyone do that.
Don't get caught in a bidding war. If there's something you want, don't immediately start bidding. Wait till it starts to slow down. If it's past your price already, you haven't gotten yourself worked up too much. If it's still in your range then bid.
If you get someone to help you load, bear in mind that the guys who are hanging around doing nothing else but loading expect to be paid. I recall a story where they didn't tell the guy ahead of time, he gave them $10, they chased him down and demanded another $10 apparently. However, the guy struggling to load his pinball will probably be happy to return the favor if you both help each other out, without forking over $20.
Have fun!
--- saint
Just wondering if I might be running into any BYOAC'ers at the Super Amusements auction in Orlando this Saturday (12/20) ...
It will be my first major auction and I am getting pretty fired up about it.
For those of you that have been, I have a question or two ...
1. I have heard that I should take an extension cord to plug machines in to power them up. Anything else you think I should take with me to "be prepared"? (off topic: Yes, I was a Boy Scout.)
2. I am not sure I am going to buy anything, but I can rent a trailer for 25 bucks from U-Haul for the day. Should I drag this big-ass trailer from Jacksonville to Orlando (about 2 hours) on the off chance that I will find something I just can't live without?
3. While I am excited to see what games will be offered there, I am really looking for a pinball machine and a jukebox that I can re-build. Am I likely to find many of these items at an arcade auction?
Any and all comments and opinions welcomed .....
Looks like it's gonna be a Merry Christmas for someone in my house this year ... though that person will probably be me. 
MM