I’ve recently bought an AIO and PSU. The PSU went back, as it had a serious fault and tripped our electrics as soon as it was plugged in, and appeared to have been previously used/fitted. OCUK gave a refund without question, so just inconvenient.
The AIO was definitely pre-used / secondhand. No box or packaging at all, and with the Intel CPU mounting only, plus no thermal paste. Luckily I needed it for a system with an Intel CPU, and it was a good price so I decided to keep it, even though I have no idea how old / used it might be.
So you take your chances with what you get.
I had been considering one of the large ultra-wide monitors, but I’m hesitant now. Can’t fault OCUK’s returns policy, but the inconvenience and risk of getting something badly faulty, rather than just something with signs of use / missing accessories, does present an entirely different proposition. YMMV.