Tag Sale Woes

We are having a tag sale this Saturday. We do this every year, usually somehow on the hottest day all damn summer. This year the weather report is forecasting a high of 77, which is not bad. Last year it was so hot we sold water! And sold out of that! Francesca is selling lemonade this time around. Our problem is that the children do not want to sell any of their stuff. We have tried being rational..."Come on if you sell some toys you will have more room for new stuff". Doesn't work. We have tried being stern - "You ARE selling some of your toys!" We have tried appealing to their love of money (especially Gavin) - "Selling some old toys will fill up your piggy bank..." - nothing is working. They see items in the boxes and they take them out, telling us that whatever it is "is my faaaaaaaavorrrrittttte toy, Mommy how can you sell that, I play with it every day" - as I brush the dust off of whatever that toy is. Gavin has gone so far as to march upstairs holding things behind his back. They wear me down. Grammy Carol if you are reading this could you come over and amuse the children in the backyard on Saturday? My mom will be helping me with the hoardes of hagglers -"Oh this is marked a quarter, can I have it for 10 cents???" - NO says my mother. I need to make sure my children are otherwise occupied so I can sell their "treasures", ha ha ha...