Since the shop does not take trade-ins, she has offered to attempt to sell your old machine as a favor to you (Her loyal customer). If you don't feel she is working hard enough on your behalf in selling your old machine, ask for it back and sell it on your own. You should be able to find the prices of recently sold machines like yours on eBay or craigslist. But I see no reason to be upset with her in this matter, as she is only doing it as a favor.