One option is to do like many on eBay. Promote the heck out of the machine, calling it "Heavy Duty", "Sews Leather" , "Goes Right Through Jean Materials". Don't forget that there is "Rare and Hard to Get" "All Metal Machine".......
Often there will be a buyer who won't know the value because it is hard to get much of the data for the nearly thousands of different machine makers and models out there.
A second thing is to donate them to GoodWill or Salvation Army or other charity thrift shop. Then you can take a write off on taxes, if you itemize and if you qualify for the charity tax write off.
A third is to try Craigs List for your neck of the woods, if there is one. Try www.craigslist.org
Maybe you can find a "sentimental collector" who will take them off of your hands.
If they are good sewers, you might let them go for not much more than shipping on one of the MYOG oriented web sites.
I currently have five machines, and if you have a Singer 401A or a Singer 500, I would be interested. But nothing else, except maybe a Pfaff 230, 332, 260 or 360......... <img src="/forums/images/graemlins/tongue.gif" alt="" />