Do you have any thrift shops, GoodWill, Salvation Army type stores near you? I think you would have less risk buying from one of these. Also check local ads in your community paper, if you receive one, the garage sale portion of your newspaper, etc. If you like garage saling, that is a good option too. I bought one vintage machine for $15 and another for $25 at garage sales I just happened to stop at. (Of course, you have to be disciplined not to buy other stuff! If you garage sale just looking for a sewing machine, you can be quick-in and quick-out.)
When I have time I check the thrift stores periodically and pick up older sewing machines for $25 to $35 to clean up for a non-profit near me. I always plug in the machine in the store to make sure it runs, and also check to make sure it has the bobbin case. If I were more concerned, I would also bring thread, scissors, needles, and fabric scraps to actually sew in the shop before buying the machine. (Alternatively, make sure the shop takes returns and save your receipt.)
I just glanced at the machines on eBay. One problem with buying a machine that way is that many people do not pack the machines properly so they can arrive damaged in transit. I am not an expert in vintage machines, but I'm not sure either of those machines qualifies as vintage. The older vintage machines have all metal parts; this is why they can be repaired forever, especially since so many were manufactured making replacement parts easy to come by. Newer machines were made with plastic bushings, etc.; those parts wear out and become non-replaceable.