When Bob explains that he got their Christmas tree for a lower price, Emily asks "Bob, how can you barter this time of year?" "Barter" means an exchange of goods. Talking the salesman down from $15 to $12 (which happened here) means "HAGGLE", not "barter". Considering that Emily is a schoolteacher AND was flaunting her IQ being higher than Bob's only a few episodes earlier (and Bob being a highly educated person) makes this a glaring mistake.