Stay away from olx/quickr/Craig’s if you are buying computer parts as these things needs be tested rather than cellphone which can verified by the time of buying.
Online forum are the best place as you can buy from trusted members who put every details like CPU-z screenshots, serial no for warranty, etc.
So beware, else ask him to come to your place or meet him at his place to check on pc.
Hope this helps.
Ps. Don’t trust quickr, that’s one lot of heaven for spammers.