Given the flux and flow and infinite changes of trade values ( and smuggle values ) and especially regarding the fact that you must be ONLINE ( Solo or Open doesn't matter ) to accept and do any kind of action in the game, the only API needed would be a ship outfitting app ( without price references as they change from station to station ).
So what kind of 3rd party tools would you consider useful ? ( plenty of trading notebooks around for crowd data )
3rd party software that everyone should try:
Slopeys trading / logging tool
Voiceattack
Facetracknoir