Sell 기능-BookSearchPage에서 추가

backend/app.js

backend/models/Book.js

backend/models/transaction.js

backend/models/user.js

backend/package.json

backend/routes/books.js

backend/routes/login.js

frontend/src/components/BookSearchPage.vue

frontend/src/components/Buy.vue

frontend/src/components/Sell.vue

frontend/src/components/login/Login.vue

frontend/src/components/login/SignUp.vue

frontend/src/router/index.js