1. This is the full tracking:
China Post Register Air Mail Tracking System

Parcel No.RS259618200CN tracking result
2016-09-22 19:40:05.0 Guangzhou international parcel process center received
2016-09-22 21:55:25.0 Guangzhou parcel process center customs scan
2016-09-22 22:52:19.0 leaving Guangzhou parcel process center ，next station Guangzhou Airport
2016-09-23 11:09:47.0 Guangzhou Transit Station export security scan
2016-09-24 17:05:40.0 Guangzhou Transit Station export customs cleared
2016-09-25 15:00:00.0 arrive Guangzhou Airport
2016-09-25 15:37:33.0 leaving Guangzhou Airport going to next airport/seaport
2016-09-30 04:48:00.0 arrive Singapore Singapore Transit Station
2016-10-03 17:35:00.0 Singapore Post Office attempt delivery
2016-10-06 14:51:00.0 leaving Singapore
2016-10-06 19:23:00.0 Singapore export customs cleared
2016-10-09 10:53:22.0 Guangzhou Transit Station import customs scan ,remark (rejected )
2016-10-10 15:51:34.0 Guangzhou Transit Station rejected ,remark (改寄)
2016-10-10 15:56:56.0 Guangzhou Transit Station import customs cleared (domestic transit )

2. As you can see the parcel is already in China. you need to contact seller and tell him the parcel is returning to him and give him the above full tracking. Apparently from 17 track this is what Singapore wrote:
2016-10-03 17:35 Address is incomplete

3. You will have to make so sort of arrangement with the seller to have him sent the parcel back to you.

Hope this help.