Im sorry, i misunderstood your question
You wanted to know how to account for purchases on behalf of your client when you act has his agent correct ??
Then you can pass entry as below
1. Purchase from outside supplier on behalf of customer
Customer A/c Dr
To Supplier A/c
2. Payment to supplier
Supplier A/c Dr
To Bank
3. Receipt from Customer
Bank A/c Dr
To Customer A/c
To Commission (assuming you receive commission)
Now if you want to have control over the movement of goods, instead of the first entry above you can pass this entry
1.1 Stock on agency A/c Dr
To Supplier A/c
1.2 Customer A/c Dr
To Stock on agency A/c
rest of the entry will be same.....