Well, sure, and like I said that rural WISP wound up failing because building and maintaining their wireless backhaul was too expensive for the number of custom

Well, sure, and like I said that rural WISP wound up failing because building and maintaining their wireless backhaul was too expensive for the number of customers they served (and they definitely did not have the budget to build a wired backhaul). In a dense area like NYC, where you can get line of sight to a building that houses an IXP, it is definitely easier to do. As for ISPs selling service, it is not all that hard to get a contract. ISPs like Hurricane Electric will sell service to anyone who can be colocated with one of their POPs, and they are pretty widely distributed (they do not provide residential service and would have no reason to refuse to sell service to a residential ISP). Verizon and AT&T also have wholesale contracts that in theory a smaller ISP could use, and I doubt they would begrudge an ISP that is competing with them in some local market (especially since a small ISP will almost certainly not be able to provide a triple-play package or any form bundling at all, so the big players will always have the upper hand). "Enterprise" transit services can also be used, if you can afford the prices ISPs demand for those contracts (or more precisely, if you can effectively pass the cost on to your residential users). You may have to build out a backhaul to some large building where you can get a fiber connection, but that is not an insurmountable barrier to entry.