The main reason they sell is the simple one: There is a sucker born every minute. People sell them to people that don't know any better. Another reason people still buy them is it's like buying a lottery ticket. There is that one in a very huge number you will find a block and with solar power you can let them run forever. They are still usable on other coins and probably testnet but I'm sure s1-s5 are heavy on those coins now.