CEX dont use a grading system for GPUs.
There is no "A" grade or anything like with phones. (Where A grade needs the box etc, B grade good condition etc)
If the card works, you get the price advertised on the website.
I have seen some pretty grubby looking cards on sale there, so not sure how bad it would need to be before they reject it.
You dont need the box or anything else with it to get the price advertised. Only exception would be like the 12-pin power adapter with the FE cards, as they wont be able to test it without that.
Where did you hear they give more for the card if it has a box? Never seen that mentioned.
I have sold them a few cards, 1 with box the rest without. No difference in price paid. It was always exactly what was quoted on the website.