If an individual is paying $600 for a EM project gun game missing parts they are already in trouble. There are very few titles that command more than $2k fully restored, down to all components and repaint. Stay away from high end auctions, as they are not guarantees of functionality or maintenance.
Consider these points.
If you have a specific title in mind, find three examples, as you may need the spare parts to make a single game. I learned this from experience.