At various points throughout the campaign, you can buy items cheaply and sell them for substantial profit and experience. An example of this technique can be found in Henderson, where a doctor sells medical supplies. Once you have the option to purchase bleach, make sure that you buy as much of it as you can afford. To the southeast, you'll find the makeshift hospital, where you rescue Nina. After doing so, head back to the hospital, where she will offer an ongoing quest. The bleach that you previously bought for $168 can now be turned in for around $1200-$2100, with an accompanying reward of around 14,000-55,000 experience. You can repeat this process as often as you like.