If someone asked me for an ink cartridge, I would assume he wanted a ink cartridge refill for a fountain pen.
I would ask him what he prefers, gel, roller ball or ball pen refill? All three are different ink systems, the roller and gel being more simular. I have not tried to find a ball pen refill for a Baron. Ball pens are more of the Parker style and Cross style. The roller and gels are more common and the more requested. AIrplane safe per Pilot, "eliminates leakage, due to air pressure changes"
As with Ed, I would include a disclaimer in you pen box, stating "ink causes staining, seller is not responsible for ink stains"