You can safely purchase a used flute to learn on and practice. However, you should have a teacher or repair tech inspect the flute's condition. Some used flutes aren't in playing condition and will cost a lot of money to fix.
But if you find a used instrument in good shape, you can save a good amount of money. The only thing you'll want to do is use some rubbing alcohol on the embouchure hole. You can also let the flute sit for a few days to make sure any bacteria won't infect you.