What determines the weapon sold by the black market weapon trader

No, while you spend time in a node (ie, repairing your craft, organizing your crew or weapons, shopping, upgrading, in combat, etc) there is no movement of the rebel fleet. They only move when you make a jump within a system.

In a non-nebula system, jumping through nebula will make the rebel fleet slow dramatically. It's hard to estimate the effect exactly, but at a glance their progress seems to be halved. I do not believe that the effect is as dramatic in nebula-type systems. If you hover over a nebula jump node in the system, you can see this on the tooltip.

There are also certain events that will either speed up or slow down the rebel advance. For instance, if you encounter a rebel scout and fail to destroy it before it charges its FTL drive, you'll find that the next advance of the rebel fleet will be much larger. Likewise, there are certain automated rebel craft that can be hacked to slow the fleet's advance.

From this page:

Humans comprise the majority of both the Galactic Federation and the Rebel Fleet. Humans learn skills faster due to their reduced XP ability, have no notable weaknesses and are the cheapest race to hire from Stores.

• Skills improve slightly faster (10%)

Engi originate from the Engi Homeworld. They take 50 to hire, with the following special traits:

• Repair speed is doubled
• Combat damage is halved

The Mantis originate from the Mantis Homeworld. It takes 45 to hire them.

• Inflict 1.5x damage in combat
• 1.2x move speed
• Halved repair speed

Rockmen are strong creatures and take 65 to hire. Their abilites are:

• Immune to fire
• Movement speed is halved
• Health is increased to 150%

Zoltan usually appear in Engi Homeworlds or Engi-Controlled Worlds. They appear rarely, and it takes 65 to hire them. Their special abilities are:

• Provides (1) power to occupied system
• Health is 70
• Explode when killed, damaging all enemy crew in the room

Slugs are one of the rarest crew to find. Their advantage is the ability to see your enemy's crew, and to be able to see the rooms surrounding the slug in your ship if your sensor goes down. They cost 45 to hire.

• Telepathic powers reveal rooms and lifeforms even when sensors are down.

The Lanius are scavengers – metallic lifeforms that rely on the absorption of minerals to sustain themselves. They are available only in advanced edition and even then are almost exclusively found in the Abandoned Sector where they cost 50 to hire. They need no oxygen and also drain oxygen from the room they are in.

• Drains oxygen from rooms
• -20% movement speed
• No damage from suffocation

And a final 'secret' race:

Crystals can be found in Life support pods that are revived by scientists. Doing so also begins a special quest to the Rock Homeworlds.

- Lockdown power: When finished charging, sends crystals to surround the room they are in, making it completly invincible to damage (Only that specific room). Nothing can exit/enter the specific room until the ability wears off.
- Reduced suffocation damage
- Movement speed -20%
- Health 125