Many people want a reliable phone or laptop without paying for a new one. A refurbishing business buys used devices, repairs and tests them, and resells them with a short warranty, online or from a small shop.
This idea needs real repair skills, a trusted source of used stock and careful testing, because returns quickly eat into margins. Keep a record of where each device came from and follow e-waste rules for parts you cannot reuse. GST registration is usually needed, especially if you sell through marketplaces.