You you owned a computer shop, it wouldn't be worth your while pawning off the old crap.
Old network cards are useless because you can buy new 10/100 cheapies for less then $20 US, and they generally work very well. Beats wasting hours configuring non-plug and play junk, and with technical support, which would cost you more in the long run.
Old sound cards are just the same, you can buy new sound cards very cheaply too (they might not be very good, but they still work fine for most people). However AWE32, AWE64 cards are still good and I would never throw one out unless I absolutely had to.
There are some really nice old cases too, but most of the old cases I have seen have been TERRIBLE to work on. The work are probably the AT minitowers. No space to work in, sharp edges everywhere, things often didn't fit together like they should, yuck. Cases have in general improved since ATX was introduced.