Who does usually sign a purchasing contract?

Depends the Authority levels for Purchase order

Example :

Purchase order for Materials or equipment

Limits1000Dollars : Authority sign : Purchase Manager , Finance Mananger

Limits2000Dollars :Authority Sign :Purchase Manager ., Finanace Manager , Project Manager

Above Purchase Manager Limit ; Purchase Manager , Finance Manager , Area Manager

Above5000Dollars : All sign  include Director .

 

It is between the customer and the supplier: From customer side: Purchasing Manager or Purchasing Officer, In some cases the other involved sides like Logistics or Top Management (depends on the contract type).
From supplier side: The General Manager or Sales Manager (Note) It is preferred to submit a contract copy to Contracts Department also to submit a copy at first time only to Finance Department for a better communication.
Regards,

MD & Supply Chain Manager

The Purchasing contract will by signed in between Supplier & Customer.
From Customer - General Manager & Purchase Manager.
From Supplier - General Manager & Sales Manager.