东西 here is 'things', with a neutral second syllable; said dōngxī, with a full tone on the second syllable, the same characters mean 'east and west'. Watch the verb's tone too: 买 (third tone) is to buy, 卖 (fourth tone) is to sell.
grammar
Other words can go in the middle: 买了很多东西 (bought lots of things), 买点儿东西 (pick up a few things).
