I agree with your assumption that 'central purchasing' is a benefit. We have done that for our two stores for about 2 years now in a different format. First, since there are only 2 shops to buy for, my purchasing manager negotiates the best price for material (parts, supplies, office stuff, service eqp. and support), gets a 'volume discount' and executes contracts for both. Each store is responsible for ordering, inventory, useage and tracking. In your case, you could probably staff a central office and do the purchasing for all your stores. Keep these things in mind:
Are all stores alike, or do they use different parts and material?
You may also create a paperwork headache with a centralized departement in the sense that for one dealership to get stuff, they would order it through your 'central' office and they in turn would bill to that dealership. Duplication...
What you describe is similar to a co-op which have been done succesfully in the past. The management of the co-op would find good vendors and negotiate volume discounts. Our stores would individually order from the vendors and the billing would come thru the co-op.
As far as the 'profit center' idea, co-ops generally operated as a no-profit system where all extra monies were rebated to the member dealers based on what was purchased.
[This message has been edited by sallen1 (edited 05-24-99).]