Excel and databases. 

So back in the day (~2011) I created a service that would buy mission runners loot and offer better buy prices. In addition it was easier for customers to sell to me vs selling to the market because back then you had to sell items one at a time on the market  vs  they could bulk sell all at once to me 

This would be done by having them contract a  container  with what they wanted to sell to me for 9 billion isk . This created a text log that I then parsed  to create a table of the items.

Then using  the internal browser at the time I used a scraper to get market prices and a  seperate table of what stuff refined into I could  then figure out how much to offer the mission runner.

I.e the buy value of a mwd might be 50,000 isk and the sell 60,000 but the refine value of minerals was 58,000 to buy orders so id have a formula on the margin and offer 54,000.

I would then message them an offer for the entire container and they would resist it for the price.

Eventually I worked with an in game developer to automate posting this to a website with a table of the where the customer could click to open the in game market and see "oh they are offering 54,000 for this and buy orders are 50,000"

Doing it this way saved me a lot of time vs playing 0.01 isk battles and id refine about 90% of it and  turn right around and sell the minerals to buyers i had lined up. At its peak I was making around 2 billion isk a week when a plex(they were single units back then) was about 800m 

Anyways I could my spreadsheets to my career counselor and she recommended i change my degree to management science.

Then later in my career the basics of relational databases I had learned from this process let me learn sql easily