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Yeah, I get it that the really old stuff doesn't sell without marketing. Do a little research to determine if any of these vintage units are old enough (1970s-80s) that they could appear on an Eroica bike. Advertise this when you make your eBay or Craigslist listings. There's always somebody who remembers paying real money for these old things and would welcome a bargain, or a minimalist who doesn't need a bike computer that displays the current GPS coordinates or even the time of day. As for me, as long as I get the current speed and trip miles, I'm happy.
https://eroica.cc/en/california/regulation
It can help to move these old cycling computers along if someone goes through these devices to determine if they are working, and to organize them into (as much as possible) complete sets that include computer, compatible mount, and sensor. Pop in the appropriate battery and see if the display comes up; to boost profit margin you can remove the battery after you complete your testing. Put the cycling computer on the appropriate known to be good mounting bracket, and see if it detects a magnet passing the sensor, or just use a wire as a jumper to connect the appropriate pins on the back of the unit. Bag up each working set, and write "Tested Working" on the bag.
Our shop periodically has a parking lot sale where things with low resale value are priced accordingly. If you price things right, you should be able to at least bring in a few dollars.
If there aren't any takers at all, they can go into a "free" box in the shop. Even if you can't sell these things, you can at least use them to reward people who volunteer at your shop.