Lady Macduff says that she will not have a problem finding a new husband, she “can buy twenty at any market.” Her son makes a clever wordplay when he replies that she will then resell them. In Shakespeare’s day, to buy and sell meant that you were deceived (as in, you’ve bought something you shouldn’t have bought in the first place, and now have to sell it). So, he’s cleverly saying that she has been deceived into believing her husband is dead. Remarking this, Lady Macbeth makes a pun herself: though her son speaks with the intelligence (or wit) of a young boy, he is, in fact, quite “witty” for his age.