William Smith of Hang Fire Books in Brooklyn listed the 16 things he keeps in his jump bag when he scouts for used books. Here are a few:
1. Electronic lookup tools: There are many variations (scanner + DB, phone browser, laptop, etc). I use a Palm Treo with Scoutpal that lets me input 10 ISBNs per query and I can check pre-ISBN title on ABE. Not state of the art certainly (and it's dependent on cell phone reception) but I haven't felt the need to upgrade.
2. Scouting Book: A collection of title lists and identification points compiled through research and experience. Two lists I find very useful are a) $$$ Titles That Book Clerks Don't Know About and b) Out of Print DVD and VHS. I keep these lists on my Treo which I sync before heading out.
9. Folding Wheely Cart: I'm fond of these models. Good wheels, folds small and carries a ton.
15. Handi-wipes and moisturizer: Few things are more disgusting than the gray and dried out hands you get after a few hours of digging through old books. If you plan to bring your hands anywhere near your face (or other people) handi-wipes are essential. I like the individually wrapped ones you find at Chinese restaurants.
16. A Weapon?: There's been some talk on the Biblio list lately about self-defense for the bookdealer. While I haven't yet felt physically threatened at a sale, I've certainly found myself in bizarre and uncomfortable environments. I firmly believe you shouldn't pack a weapon that you don't want turned on you, but maybe learning a little book-fu is a good idea. An OED would make a fine bludgeon, pocket books fly like throwing stars.