See these threads:
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/90074/tips-for-new-players/p1
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/149536/kwik-e-mart-also-known-as-kem-farming-guide#latest
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/101056/house-prices#latest
Tapping Sideshow Bob in Other Springfield can make quite a lot of money in a short time:
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/205386/sideshow-bob-flaw-and-order-walkthrough-and-tips#latest