客人向店員投訴問題

一邊翻書,一邊向店員投訴

Both actions (翻書, 投訴) should be in continuous tense to show they happened at the same time.

翻書 = flip through the book
投訴 = complain (to somebody) (about/of something)

一個客人一邊翻書, 一邊向店員投訴二手書上有鉛筆跡
She was flipping through the book while complaining to the shopkeeper that there were pencil marks all over the used book.

If you want to use the verb "frown", you can use the express "to frown at sb./sth." (對某人／某事皺眉頭)

She was flipping through the book while frowning at the shopkeeper complaining about the pencil marks on the used book.

2015-07-18 12:18:24 補充：
typo: the expression "to frown at sb./sth."

2015-07-18 12:24:22 補充：
Note: Without reducing the adverb clause, the whole sentence could be written as:

She was flipping through the book while She was complaining to the shopkeeper that there were pencil marks all over the used book.

嘩，你照字譯，同大陸人一樣噃

the customer is complaint about the pencil marks found inside the old books

呢個投訴當然無聊，你可以話nonsense。