From reading this thread, I'm guessing two things:
1)The conductor has the power to sell rooms at the lowest bucket price on board, but even if there
are rooms free when you board, that doesn't mean they won't be filled by someone getting on later.
The only way the conductor knows if he has a free room between points A and B is to call the home base,
and that takes a little time and energy that not all are willing to exert.
2)You can try to talk to a ticket agent beforehand, but it sounds as if they don't have any authority to offer
a better deal than what you'd get by calling in a reservation the day of departure. Your only hope is that
enough reservations cancelled just before departure to drop you to a lower bucket, or that the agent
knows how to sell you a cheap compartment, which might be a bit outside the rules.
Others have mentioned elsewhere that Amtrak will sometimes preemtively call its frequent travelers
who are booked in coach, and offer to upgrade them to a sleeper if space is still available close to
departure. I'd guess that most sleeper travelers make their reservations far in advance, and the chances
of selling additional sleeper space at the last minute are pretty close to nil, so sometimes Amtrak will
offer last minute deals. My understanding is that these prices can go below the lowest bucket price.
Actually, the Conductor has a manifest that shows all passengers and their origin and destination, so they know what sleepers are available and which are already booked for downline use.
The ticket agent can sell only what is in the computer system and generally cannot offer a lower price. They do not have the ability to override the pricing without a rational explanation - and if they do something they should not, it will come out of their pay, so don't ask them to do something that will get them punished in the long term.
The best way to get a deal on a sleeper is, just as has been described, ask the Conductor and make a deal.