You wont be able to sell bot parts (we use to make item addons out of) then but
they are actually  quite nice form of income.

We should not be able to sell anything to a vending machine imo.

Other example: stone.  Since stona already sells some nice variety of items
(boots, weapons, shields) I do not have any problem with her also buying drinks
or books or addons.

As Xenux suggested on irc, it may be better to group items and specify groups to
a shopkeeper.