Actually, I think it has some potential. Paying through mobile phones is huge right now and I'm sure there are businesses out there that might be interested in this. Larry is right when he said that .co is a pretty useless extension BUT you might get lucky and find a buyer. Plus, a lot of people find these apps through the Google store rather through Google itself so it'd be easy for a company to list this as their website under their app page. It'd be worth listing on Sedo but in terms of the price, that's a tough one. You might just want to do the "make an offer" option for this one.