We should be able to sell items we have used

[Felis]

I know where the binding item mechanic is coming from, to prevent hackers from making a damage to the stolen account, but do we need this anymore?

To remove gears from the market and to prevent oversaturation.
That was a problem during 1.0. As a solution came the materia system, that removed gear by spiritbinding and destroyed the whole gear if a meld failed (not just the materia).

I think it's supposed to be a gil sink.

The right term is gear sink