any thing that you want to be sold/ shown as an equipment must be declaired in the apropriet ****_goods.ini file
but i was experimenting and it seams that in whatever "goods.ini" file you put the equipment it works so try and declair the stuff in the "goods.ini" file
then in the "market_misc.ini" to put them "in stores"

* the engine_equip.ini is for the engines status stuff, thrusters are in the st_equipment.ini for stats, and in st_goods.ini for declairing them as goods.

thanks