Confused about raising money? Ticked off mom?

I am going to New York with my mom, sis, and brother-in-law, later this summer.We are all adults (over 21) My mom and I are scraping the money together to go. So I found the beer trays that we were going to sell at our rummage last summer and put them on Craigslist for 40 a piece. We were going to sell them at the rummage for 10 a piece. Someone replied back who seems interested and now my mom is all upset-she is acting like they are family heirlooms when they are not! They are actually worth less then 40 dollars each. We are a family who is barely scraping by there is no sentimental value to these at all-none-nada-zip. How can I convince my hoarding mom to let this stuff go!?