It's all about the price.  Ideally they'd all be bought new/sealed, but I won't pay big bucks for cellophane and instead settle for varying levels of less completeness:
Turbobs:  they generally have to the game, case, and manual.  Stuff like spine cards and boxes are nice but not required, and I'd settle for even a loosey if it's prohibitively expensive to get a more complete copy.
PC-FX:  ditto, though you rarely see 'em as loose anyway.
PSP:  it's not OBEY, so I give less of a shit.  They all need a home, but I'm okay if they don't have the manual or are in replacement cases (made by me); the Ys games were an exception, though, as those had to be complete and the fancy versions for the three from xseed.