I recently came across this neat little site called Flippiness.
The basic idea is, they run some software that checks the prices of books between different websites and determines if there is a discrepancy in price. Each day it calculates pricing to find profitable items and lists them on the Flippiness website.
You buy the book from the cheaper website and you sell it to the website that is offering more.
You can do this all day flipping books for anywhere between $1 to $20 bucks per flip. Flippiness claims that several of their members are doing this full time and earning thousands per month in profit. (Your results may vary)
This probably will not make you rich, but it might create some cashflow for you to be able to afford advertising for your other businesses.
They have a free plan if you just want to test it out before you go all in. Click here to take Flippiness for a test drive.