From The LNR Book of Knowledge

Here is a list of stuff not to vendor off as it will fetch a good price on the exchange:

• Any Admiral rank weapons
• EPS consoles
• Plasma Torpedo Launchers
• Turrets
• Tetryon/Polaron Ship Weapons
• 72% Food. Some of them go really expensive in the Exchange
• 'Sensor Probes' science console that boosts Sensor Array (used for Viral Matrix/Scramble Sensors)
• 'Induction Stabilizer' science console that boosts Deflector Field (used for Science Team/Feedback Pulse/Dampening Field)

Many of the common base items for 'crafting' Memory Alpha are of extreme value!

• Quantum Mine Launcher mk IV and VI
• Plasma Torpedo Launcher mk IV and VI
• Plasma Beam Array mk IV and VI
• Tetryon Beam Array mk VIII
• Polaron Beam Array mk X
• Covariant Shield Array mk IV, VI, VIII, and X

Etc.

Lots of the base items sell for more than the rare item you get after the combine! Everyone wants to skill up.