why is the se necessary? What does it refer to

Question

La tienda devolvía el dinero ________ recientemente.The shop gave refunds as long as the item had been bought recently.

Inma Sánchez · Accepted Answer

Hola Joanne
The "se" in "se hubiera comprado" is the passive mark. We have a type of passive that is more "relaxed" than the more formal passive with ser + past participle. 
In this case it means: ... as long as it had been bought...
Here's a lesson on the "pasiva refleja" or "se passive".
I hope it clarifies it.
Saludos
Inma

Joanne Ogata · Answer

This the answer from Qwiziq to the above which I omitted to include:  " mientras el artículo se hubiera comprado"  However I still  don't understand why "se" is part of the answer.

Joanne Ogata · Answer

This the answer from Qwiziq to the above which I omitted to include:  " mientras el artículo se hubiera comprado"  However I still  don't understand why "se" is part of the answer.