i have the
First of all, make sure YOU own it and that it's yours to sell -- with insurance sometimes this is tricky, even if you've had the thing for a couple of years and no one's ever bothered you about it. Just check to make sure it is completely bought and paid for, etc.
If it is, ebay is an option (I've seen them on there before), but keep in mind that the Vest is a prescription device. Technically it's not supposed to be transferred to someone for whom it wasn't prescribed. Also, the vests on ebay (even newer models) tend to sell for pretty low amounts.
Personally, if I were interested in selling my vest I would contact my clinic and ask if there were any patients in need of a vest who were struggling with insurance issues or other red tape. Your doctor/nurse might be able to help make a match that works out as a deal for both of you, and of course you'll know it's a legal transfer to someone with a need for the machine. The other major upside is that your clinic can help sterilize the machine and you avoid shipping costs.
I would definitely start with your clinic and move from there.