Well, item quality doesn't need to be very involved. A simply flag (0 for new, 1 for used) would be enough to distinguish between new and used items. But yeah, trade-ins would be a nice touch even without that. 
But then you also need the effect of new vs used. We have items that provide a +1 bonus, so now do we need the system to handle fractions? Or are second hand items simply more likely to break once that is added in.
