As my collection grew, I found I kept machines that do a certain thing and do it very well. I have sold many machines over the years. Where I list them depended on the demand for the machine and the condition of it. If it was a high demand machine in great condition with all of the accessories I listed it on Ebay. If it was a good, basic machine for someone starting out collecting or needing a working machine I used Craigslist. If it was a 'Barker' with bad cosmetics and no accessories but in good working condition I'd either donate, Freecycle, or list on a local classified board at $20.00 to get rid of it.
The machines I have kept may not be 100% perfect but for whatever reason they have survived many purges of machines. I have kept a 1934 Singer Featherweight, A 1942 (If I remember correctly) Singer 15/91. A Black Singer 301in horrible condition (it is a work in progress), a Pfaff 1222E with all its accessories, case and manuals, and a Pfaff 6120 mechanical from the early 90's. The German built Pfaff 6120 is the machine that was the turning point for me regarding letting machines go. It has everything I need in a machine for everyday use - needle threader, built in walking foot, excellent straight stitch, zigzag and straight stitch needle plate, 10 different feet, and being mechanical it should be around for quite a while.
Do I miss the machines after they go to their new homes? No. Well - only one, a Singer 404 straight stitch machine. She just purred and if it had not been for the horizontal bobbin and no truly drop-able feed dogs she would still be here, I even have a card table I would put her in. If I ever get the Black 301 up and running It will be able to use the card table.
I have made money on all the machines I sold - not a lot but I didn't lose money. I even shipped a Pfaff 130 to California via Greyhound bus. I used a big plastic tote, 2 new bed pillows and some bubble wrap to pack it. It arrived just fine. The great thing about using Greyhound for shipping machines is they don't switch buses, they switch drivers, so the cargo on the bus never leaves the hold until it arrives at its destination. It is also cost effective for heavier items which the Pfaff 130 definitely is!
Hope this answered some of your questions.