How do trade-ins and buy-backs work?
Buying a handset over the counter is recorded as its own transaction: seller details and ID, a condition grade, and what you paid. The handset then becomes saleable stock tracked by IMEI, so the margin is visible when it sells.
Buying from the public is not the same as a sale in reverse — it needs an identity record and a condition assessment, which a generic till has nowhere to put.
Keep in mind that the identity documents you capture are personal data you are responsible for. Your own privacy notice should say why you hold them and for how long.