There are several large vintage groups on Facebook. Some allow selling, some do not.
I would suggest "Vintage Sewing Machines" (17k members, excellent reach) and "Vintage Sewing Machine Swap & Shop" (4k members). You have to be a member of #1 to join #2 I believe.
I've sold through both of them with excellent results. I've had local people come pick up treadles and a few people who were willing to pay for good packing and handling. I found a buyer through there who was completely willing to pay for two double boxes for machine and bentwood.
Good luck.