Build a POS (Point of Sale)

Hello, Only The Professionals i need POS (Point of Sale) Website or / software (Any) POS Point of Sale our company need something to manage our STORE STOCK and Sale products. POS For Sale Mobiles Phones / Accessories / we have diferent prices for same products. Example: We Buy iphone 6 at 500 USD and set in pos for sale 600. if next day we buy iphone 6 at 400 USD and set in pos for sale at 500. so what to do in this case? Same Product = Multiple Buy/Sale Prices. which product we buy cheap we will sale cheap and which we buy expensive we will sale expensive and and end get a Correct (Income Report).. We Only Need to set multiple prices for the (Mobile Phones Catagory) other things has fix prices. buy / sale. so about phones as my idea. we can set it in POS by (IMEI) every mobile phone has a UNIQUE imei Number. so when we add a product in mobile phones example: (iPhone 6) so we will need to add Sub Product of iphone 6. and in subproduct we will add the imei numbers. so when we add the any imei number as sub product so we set the price For that imei number. so in the sale option. when the seller will sale. in sale search option. he will write the imei number or unique number to find the product. about stock. on mobile phones Main Products stock will depend on (SubProducts) if 1 main product has 10 unique number saved as sub products so it mean it product has stock the 10 units. Requirments: multiple roles for admin seller vendor. debit or credit sales. intigration of printer / cash drawer credit of customers. credits of venderos. STOCK inventry. easy to use. sale report (daily weekly monthly or datewise) income report (daily weekly monthly or datewise) LOG of every activity of sale or any change. sales man reports. which sale man sale which product and daily how much weekly and monthly to check progres of sellers which is the BEST seller. we dont need any crypted software of website script. WE NEED To Finish With the Fastest Way. not weeks or months.