Maybe you could sell pen refills.

It was at this moment that Don realised that the brass pen refill had some form of potential to absorb impurities from water, in this case, chlorine.

The shop also has an extensive selection of fountain pen inks and cartridges, including colors like purple and mustard, ballpoint pen refills and pencil leads.

Out popped a pen refill ... a metal spring... and from the back part of the pen: a clear plastic tube filled with wires, a miniature battery, and a tiny transmitter.

Standard Space Pen refills can be used in any pen able to take a standard Parker Pen Company ballpoint refill, using the small plastic adapter that is supplied with each refill.