Not true. I bought and sold many different items on Facebook Marketplace (cars, motorcycles, furniture etc) and everything was okay. I have never been cheated on.
Immigration allows you to leave the country with a re-entry for no longer than 2 weeks, once a year. If possible you should use the school holidays. If you travel around they will deny to enter the country because it looks like you're misusing your visa.
@Dean Robert Hebron: There are many language schools that are closed by the authorities every year because they do not comply with the regulations and offer ED-Visa online classes which is against the law! For an ED-VISA students have to attend classes. That's the fact!