The Department Buyer should be considered the "Purchasing Authority" in your department. This is the person that knows their way around your funding, object codes, restricted items and purchasing guidelines. This role will really be the final set of eyes

Your Department Buyer can check the order in one of two places:

PRE-Approval, which means that they are the final set of eyes and are making all of their changes before your Account Approver is signing off on the funding

POST-Approval, which means that they are the final set of eyes and are making all of their changes after your Account Approver signs off on the funding.

This is really up to your departments best practice and what you feel works best for you.