For older machines this is what I would do IF I needed to sell/get rid of something:

1. Rare and valuable machines would go to ebay , if the value isn't a few times more then shipping it is not worth bothering.

2. Common machines with uncommon upgrades would get parted. The upgrades go to ebay while the machine itself would be sold locally or freecycled depending on the condition (return it to stock if you can). Non or partially working gear would be parted out if people showed interest and the rest would be recycled.

3. Trade. Somewhere somebody has some junk you would gladly trade for your junk and both would end up happy in the deal. If it is local even better. Some people live for this. A decade ago a guy I knew traded a crappy K6-2 machine for a crappy small boat. I was there when the trade was made and both people looked like they had screwed the other person on the deal, funny to see when you are not involved.

You can replace ebay with computer forum forsale sections depending which way you are leaning (to a good home.. or just want the most cash).