Testing used pc parts without a PC

I'm planning to buy my first PC, around £1,400 budget. I've found some good deals for used parts but if I don't have a PC to install and test these parts, how can I know if the used parts are functional? Will I just have to buy everything within a time frame and build then test them all ASAP? Would it be safer to buy the bulk of the PC new then only buy the expensive stuff CPU/GPU/RAM used and then test? HELPPPPP