They are so cheaper than HD or Lowes, and basically have same materials with better selection.

I can see the logic in this, and I am not trying to be a control freak, but I want to put common sense things in the leases like no flushing diapers down the toilet, not storing abandoned vehicles on the property, not storing items in the common areas, no hazardous materials, etc, and recovery of expenses related to various issues. 

The one thing I am thinking is, you expect the contractor to pickup materials when you want labor only contract, that's a killer for me, if you let me bid on labor, i expect you to deliver everything to the site when i need it, any materials that will delay my work, i will charge you.

My purchase agreement includes all materials and equipment on-site related to the maintenance of the property.  

Since this is an investment property to serve as rentals I am looking into how I can write off the building materials for my US taxes.

Do you have any advice or reading material that you would recommend to dealing with the eviction process in the situation as well?

Real estate is proforma driven and value is based on the current and proforma NOI.

I was thinking way less hourly. on your 15-20% is that on contractor bills as well as materials?

I'd really like to give back part of it, but we incurred some hard material and labor costs due to their negligence. 

Check with your lender but you want to hire your contractors on your own and promise them payment as they normally receive (when work completed or you buy materials and they provide labor).