OK so I collected an Assassin. Do I sell it? Repair it?, use it? deliver it somewhere?

Some mech collection jobs require a specific variant of a mech (others allow any variant). Assuming you either have the correct variant or a non-specific job, there will be an extra popup in green text (the same as turning in any other completed cantina job) on the main obital view, where you'd go to the mech market, select a contract, etc.

As mentioned, some require specific variants (wrong variants don't conribute anything), and also that non-specific mech collection jobs will also accept hero variants (so be careful not to hand over the wrong mech). Next, the condition is irrelevant - a wreck is as valid as a fully repaired mech. Finally, jobs can be turned in anywhere.