Can relationships have relationships?
You want to avoid any inconsistency in prices - and you have already identified this in your question.

I'd say use a "Pricing" table which stores Supplier-Parts(PK) and Prices(Float or Decimal value etc).

That way, each part from a different supplier has a different price. In other words, the price depends on what the Part is AND who the Supplier is (Supplier-Part is the Primary Key or Unique Index).

The Purchases table should just include records from this new "Pricing" table - which links to all the info from Parts, Suppliers and Prices.

Makes sense?

EDIT: I just noticed. You are relating Parts and Suppliers again in your Purchases table, when you already do it in your Sells table.

Just have these 4 tables:

Parts: PartID (PK), PartName
Suppliers: SupplierID (PK), SupplierName
Pricing: (PartID,SupplierID) (Unique Index of these two Foreign Keys), Price
Purchases: PurchaseID (PK), (PartID,SupplierID) (Foreign Key), Quantity