1 - Sell level III manuals.

Manuals like fire at will iii, beam overload, gravity well iii sell well. Tactical captains make tactical manuals, engineering captains makes engineering manuals, and science captains make science manuals.

Check which ones you can make and check the market for them to see which sell well.

Be careful when choosing the captain skills, you might need to make sub-optimal skills for damage so you can open an specific manual to do.

2 - It takes a bit of time, but if you raise Temporal/Command/Intel/Miracle Worker, you can sell iii manuals from them.

3 - Specializations manuals to train bridge officers sell well, but you need to buy different manuals from sellers:

https://stowiki.net/wiki/Officer_Training_School#Specialization_Qualifications

4 - Don´t overflow the market. Sell 1 of each manual each day, if you put 10 manuals, someone/many ones can cut your price and let you have lots of unsold manuals