Services for Products
Agree with the above and I would also add several suggestions: 
 - use Taxonomy and Taxonomy Access Control modules to control access to
   content and who can update which service node
 - use Computed Field module that would take a value from related content to change a value (price) of the service/order depending on who did the work and what was the final price

Also, maybe this problem should be approached from a different angle? Maybe the service "product" should still be sold through e-commerce but instead of being "sold" would be "ordered" first, service performed and only then the final price updated, sold and ready for payment? Possibly that would require adding extra step in the checkout or allowing people to "order service" (Service would be a content type that people could assign to themselves as the "buyer") and only when performed and price was assigned, the Service would go through regular e-commerce checkout process with payment mechanism.