feat: complete sell

lib/data/repositories/book_instance_repository.dart

@@ -23,4 +23,12 @@ class BookInstanceRepository {
   ) async {
     return await _apiClient.sendBook(book, owner, bal, price);
   }
+
+  Future<Result<void>> sellBooks(List<BookInstance> books) async {
+    Map<String, double?> res = {};
+    for (BookInstance instance in books) {
+      res[instance.id.toString()] = instance.soldPrice;
+    }
+    return await _apiClient.sellBooks(res);
+  }
 }