卖 mài to sell; to betray; to spare no effort; to show off or flaunt
买 mǎi to buy; to purchase
If you get the tones of these mixed up think of the shop assistant who wants to SELL you something, gets it and places it down on the table. That's it. It's sold, one movement down.
If you want to BUY something, you take it to the counter and put it down, pay for it, and then lift it up to take it home .. down and up.