Currently, all available Avionics can be traded amongst players.

Can you trade Railjack Avionics?

Yes, they are tradeable.

How do I upgrade my Avionics in Warframe?

Outside of the grid, Avionics can be upgraded with Dirac like regular mods. Click on the Avionics tab next to Grid and you'll be presented with a list of all your equipped mods. So long as they have an open pip they can be upgraded, but the cost does increase with each additional upgrade, much like mods.