Quality Control is a difficult thing for any business. Even though you may not be producing these items it directly reflects upon the business. It's what the customer knows and where they purchased a product.
1. Speak Up - Don't feel bad saying something if it's not up to par. It needs to be to sell products and to keep customers loyal.
2. Get Rid of it - If the product isn't as great as the others then stop selling it. It just makes the other products look bad too.
3. Test Products - See what you can do in your shop, drop the product or whatever the case may be to see what the customer will see.
Make sure your quality is always at it's best and customers will notice. They have no problem paying a little more money for a superior product.