in fact, they are not so much different. all the meaning of 售 and 卖 is sell. sometimes we use they together售卖, also means sell. 卖is more popular, you can use everywhere, and usually use in when you are speaking you can use when you are selling anything. 售is more official, usually use when you are write it.

卖 oral 售 formal “卖东西”也可以说“售货”。 “卖衣服”也可以说“销售服装”。 “售”is often used in some jobs,such as “售货员”，“售票员”，“销售员”，“售楼员”， it sounds polite.

@sheleven8: I have already successfully passed my HSK 3 and now prepare for level 4! @yiqi: Thank you so much for your clear explanation ;) !

you can say 他在卖东西 but can not say 他在售东西，if want to use 售here, should say 他在出售东西。售can not just use single word in sentence but 卖 can.

what's your HSK number?