I have bought almost all of mine off either eBay or Craigslist. The others I know of are etsy- but I rarely look there, and Goodwill (which you could not use to sell). I'd like to know if there are others too - because I still have a few machines I'm searching for.
Best way to find out what they are worth is to try to sell them. If you get no takers, you are asking too much. You can look at completed sales on eBay (not active listings, what the seller hopes for and what they get are often far apart). Craigslist - you have to watch and write down prices and then see how quickly they are removed. Note how long ago active listings are posted. Ones that are still there a month later may have too high a price . . . ones in demand at acceptable prices will be sold and probably deleted in less than a week (though some sellers never delete their adds after the item is sold . . .)
If you want to sell on Etsy or eBay and do not already have a reputation as a reputable seller, you can expect that what you sell won't go for as much as what a seller with a good track record can get.
I am dreading the day I need to worry about disbanding my collection. I'm hoping it won't be for another 10-20 years, and then I'll be glad if they go to good homes and not to the junk yard.