It depends in which kind of marketing you are interested in. How u win ur costomer currently?
You answered your own question when you said "I built it for me, but feel that it has commercial viability for many others." Other than what you said, what problem did you design it to solve? Who are "many others"? What problem do they have? Why not cruise the Excel forums on the MS support s...
Please clarify ;) the situation : - do you have a contract with developers ? - do you own the app ? or does it belong to the developers until you pay them ? - why are the developers asking payment ? - do you have a running app ? which needs does it covers ? - did you set a company ? - who are you...