Maybe it explains this somewhere in the Rotohog rules, but if so, I wasn't able to find it. Luckily, the guys at Rotohog are very responsive and answered my question about it. The easiest way to explain this is with an example.
Billy Wagner and Mariano Rivera are both trading at $45. Wagner has a day game today and Rivera has a night game. I have Wagner in my 'lineup' until the end of his game, and then sell him and buy Rivera with the money.
Question 1: Can I have Rivera play tonight, effectively re-using the same money for a second player in the same day? Yes.
Question 2: Can Rivera fill the same roster slot that Wagner was in, effectively letting me have more than 15 players play for me each day. No. I need to use a different roster slot for Rivera if I want him to play today.