Very simply, they need:

1. A fully-functional script. Most of the unbuyable ships do not have fully-functional scripts- i.e., they don't have all of the entries for things like cameras, mounting/dismounting hardware, etc., etc. that player-piloted ships need. Not too hard to do- just look at the ships in the current version of the XML Toolkit mod (1.3) for practical examples of this.

2. They need a proper set of Good entries, for the Hull and package. Again, see the entries in the Toolkit Mod for guidance.

3. They need to be sold somewhere, by putting the proper entries into market_ships.ini.