Used device purchases need a different workflow from ordinary stock intake. The shop should know who sold the device, what was checked, what was paid and when the item became ready for sale.
Useful records include seller type, seller contact details, purchase price, model, storage, colour, IMEI or serial number, grade, condition photos and proof of purchase where appropriate.
The safest workflow is to keep the purchase record separate until the device is checked and marked ready for sale. At that point it can be connected to inventory, labelled and sold through POS or the shop website.
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