Merge branch 'sell_YJ' into buy_with_email

backend/app.js

backend/models/transaction.js

backend/models/user.js

backend/routes/books.js

backend/routes/emails.js

backend/routes/login.js

backend/routes/transaction.js

frontend/src/components/BookSearchPage.vue

frontend/src/components/Buy.vue

frontend/src/components/Sell.vue

frontend/src/components/login/SignUp.vue