Electronics: IMEI, repairs and trade-ins

Phones are tracked by serial or IMEI: registered when received (or from the Excel import), chosen at the sale, printed on the receipt, the invoice and the sales register, refused if already sold.
Repairs are paid jobs: the ticket, parts from stock, ready, collected as one sale; counter warranty claims.
Trade-in: buy a used device from a customer in one step; it enters stock at the price agreed.