This looks great! Can I suggest you add a manifest.plist file? Something like this:
identifier = "oolite.oxp.stranger.MineralStoreReset";
version = "0.4";
title = "Mineral Store Reset";
required_oolite_version = "1.77";
description = "Randomly resets mineral store for main station to avoid market saturation.";
category = "Mechanics";
author = "stranger";
license = "CC-BY-NC-SA 3.0";
This will eliminate a couple of warning messages in the log file for Oolite versions 1.80ff. I'm assuming you'd call this "Mechanics" but feel free to change it to whatever you think is appropriate.
Suggestion: What about Rock Hermits or other stations? Should their mineral store get reset as well? It's harder to check for the same condition you're using for the main station, but worth thinking about.