1. Time. Buyers, especially buyers at national accounts, are extraordinarily busy people who can't afford to spend more time than necessary on any given task. This means that if a sales rep has 100 titles to sell in and the the buyer only has 90 minutes free to meet with said rep, a few titles will get a minute or two of attention and the rest will get less. Sad, but true.
2. The publisher's goals. When your book is first run through the amazing P&L machine, a rough print run is figured out. This number is further refined in launch meetings, print meetings, and sales meetings, after which the sales reps make their estimates based on these numbers, previous experience, comp titles, and so on. Long story short, though: unless you get phenomenally lucky, a lot of the information necessary to figure out the buy at any given account is sort of baked into the infamous P&L.
3. The American people. If you were offering a teenage vampire romance two years ago and you managed to score representation and a deal with a publisher, chances are your print run and account buys were larger than someone offering male ennui. If The People want what you're selling—or if the rep and the buyer think The People want what you're selling—it'll obviously factor into the sales call.
4. Media attention. Will the book be featured on NPR? Was it a hit at ComicCon? Does the author have a crazy-famous blog? The list is endless. Some things (like the oft-cited and impossibly nebulous "national media attention") aren't going to be of great help in the sales call, but if the book's going to be on Good Morning America or the author is an Internet celebrity, that can change the game entirely.
5. Co-op program cycles and minimums. Many co-op programs run on weekly or monthly cycles, which may not jibe with the scheduled on-sale date of the title in question. (Occasionally on-sale dates are altered to accomodate co-op promotions.) Additionally, some placement (especially those coveted front-of-store tables) require a minimum buy, and if the sales rep can't justify a buy for the account at a promotable quantity, the account won't promote the title and will lower their buy accordingly.
Tomorrow: part two!