I prefer buying from people with full box and accessories.
The feeling when you get an item with its full packaging (yes even 2nd hand) is much better (imo) then just getting the item itself. It just have a better psychological effect.
That said I do keep all my boxes as well. Some things maybe don't need to keep say monitor, case boxes and those boxes that look like just an everyday box or something. For everything else, I just keep them. Unless you own a few rigs you shouldn't have that many no?