She looked at every item in the store one after another.
Literal
She [topic-は] store [in-に] exist all [possessive-の] things [object-を] one-after-another looked-at.
次々に ('one after another / in succession') is an adverb expressing a rapid sequence — she moved through items without lingering. 店にある ('that exist in the store') is a relative clause modifying すべての物. The sentence conveys thorough, systematic browsing.