The only Sheetz location in Durham is in southeast Durham county (near RTP). There is most definitely a market between that location and the next closest location (Mebane, NC). A location in North Durham or between Durham and Chapel Hill would serve a distinct set of customers that include commuters, college students, and drivers along a busily-traveled stretch of I-85.
This fall we were in South Carolina. They are in dire need of Sheetz down there. As much as I loved Myrtle Beach, I needed my Sheetz. Stopped at the first one we found half-way up 95 in North Carolina. Vacation without Sheetz is culture shock to the extreme.