I'd come up with interesting situations and encounters that would be tavern/inn centered and start by making that the basis of the campaign - at least while the PC is actively taking a hand in running the establishment (before he goes off to adventure, leaving a trusted manager home to mind the bar).
I'd base my handling of the business of the bar on the description of the Profession skill in my 3.5 PH (or the 3.5 SRD), and largely wing it with ideas that feel reasonable and draw a lot on what I remember of Quark's bar on DS9, Cheers, and the stories of my aunt and uncle (who used to own a bar).