Help with designing sales and inventory management system
Your Product table shouldn't store the Price at all. You need a separate table called something like Purchases which has a row for every purchase made, the Price and PurchasedDate, and a foreign key field to reference the Product table.
Also, tables like Purchases and Sales are normally broken out further into two tables each. One table having header information and the other the line information. For example, PurchaseOrders would store the header information like PurchasedDate, PurchasedBy, maybe the Company purchased from (if only one Company can be on one Order), where the PurchaseLines table would be a row for every item purchased on that PurchaseOrder, with the Quantity (per item), Price, and foreign key field to relate back to the PurchaseOrders table. Similar structure for SalesOrders and SalesLines.
PurchaseOrders are one-to-many PurchaseLines. (SalesOrders are one-to-many SalesLines.)