During your time in Eve Online you will accumulate modules like weapons, afterburners etc that you may not need for ship fitting needs.
Turn those modules into ISK!
However be wary of how you dispose of them. Modules and the market do not necessarily follow the rules of rhyme or reason. Grab twelve modules, high, mid or low slot, random ones, select reprocess on each one (do not reprocess them at this stage however) and note the amounts and types of minerals. Then note the sell price on your local market for the module. Interesting isn't it?
Fire up the spreadsheet you created previously for manufacturing, invention, and make a new simple page.
Insert fields for the current immediate buy cost of all required minerals that are easily obtained in a few jumps, a quantity for multiple modules and a total.
What you are trying to do is work out the worth of the minerals that you can obtain from the reprocessed module as opposed to selling that module on the market.
Sometimes you will see the minerals are worth more than selling the module on the market, therefore reprocess the module and use or sell the minerals, which are often easier to sell than the module. Other times the module is worth much more on the market than the minerals it contains, therefore sell the module on the market. The third option that can appears leans neither one way or the other, the choice is yours in this situation, sell the module or reprocess it is up to you.
Now the interesting part.
Often people put various modules on the market for sale at less than what the minerals it contains are worth. What that means is the following. You could buy a module, either on immediate buy or a long term buy, reprocess it and then sell the minerals for more. Profit!
Which modules you can do this with is for you to find out.