I got our snake from Craigslist too. The prior owner was clueless, as was I, so I continued what they were feeding: 1 hopper mouse a week. Then I discovered this forum and learned that hoppers are for hatchlings, and that a snake several months old should be on much more substantial meals. I increased the size of the mouse, then went to 2 mice per meal, and our thin snake filled out nicely.

Having a gram scale helped tremendously. I go according to prey weight and it's worked out well. Good luck to you!