Well, the first thing you should do is lose the attitude. Telling folks what they DON'T need is no way to get them to buy an old, used, albeit vintage machine. If they are just learning to sew, they are just learning about machines too, and telling them what is GOOD about a vintage machine is a better way to go about it. Think about negative campaign ads and how we all hate them. Then YOU use it. Have it set up not just to demonstrate, although that is good, but so you can use a vintage machine on a regular basis - a project always going. Seeing you make really cool things with a vintage machine is a great way to sell it. Perhaps pick one, and focus on it till it's sold and then go to the next one. And make sure you convey that you LOVE sewing with a vintage machine. If beginning quilters see they can do all they want with a cool, vintage machine, and that there is a reason to have one ( other than " you are just learning and don't deserve all the bells and whistles yet") they will feel more like owning one and less like you are trying to unload an old machine on them.