I usually just report abandoned bikes to the city or campus police. I have never actually wanted one of them (they are usually rusted out Wal-Mart "mountain bikes"), but they do tend to accumulate on college campuses and take up all of the lock up points. That seems reason enough to ask that they be removed. I don't think I would ever take matters into my own hands, but if I was interested in an abandoned bike I think I would ask the police if there is any way to buy or just have unclaimed abandoned bikes.