Before anything else, check a used Mac for iCloud lock, firmware passwords, and MDM. The ideal situation is one where the person selling the computer has wiped it and done a fresh install.

Founded in 2014, SellGPU has built its reputation on trust, integrity, same-day payments, and a process that has been refined to reduce friction for customers looking to sell their used computer ...

Electronic stores or discounters often advertise cheap notebooks and PCs. If you want to make a decent saving (and do the environment a favour), you should know how to find a good second-hand model.