Selling it if it doesn't work should be okay as long as you indicate that in the listing.
Another suggestion for disposing of it--if you can't sell it--would be to contact your county or city solid waste department. If you're in a more metropolitan area, they may have the facilities to properly dispose of your old PC. If you're in a rural area, it's best to contact your county landfill to see if they have an annual drive for discarded technology (I know ours does). You may have to hold on to it for a bit, but you'll be able to feel better doing your bit for the environment.
Dont donate it to a school. Please dont, My old school had a few old ass IBM's donated to them and we hated the school for attempting to have them work in our tech labs. You're only pissing the students off when you donate shitty stuff, seriously.
If it's so old it doesnt use standard pci, agp, motherboard or anything else it's junk. unless some one wants to replace a shot 16 mb stick of ram in their IBM.:laugh:
Thanks. I'll sell them to E-Bay, if possible, together with my FamiCom (coz Algo Fonix said that old computer game consoles are collectable items), once I'd put up my own bank account that supports PayPal.