I didn't have time to ask for it back. By the time I realized what had happened, they had gone on to another street. I was too busy trying to figure out what happened, and exactly what I lost. Luckily, I had a cached browser tab opened on the auction page and so, through the power of deduction (logic!), I was able to see which bag had now gone missing. The game only told me "gave bigger blue bag" rather than the name I gave it, and I had a couple of blue bags..
Thankfully it was mostly food ingredients and crops that I harvested myself (a lot of meat and cheese), so it could have been worse if it was a bag of tools or emblems or something like that. Still, I was more upset with the game for letting it happen. It's funny, when you're in your mailbox, the game asks you "Are you sure you want to delete this message?!" but there's no "Are you sure you want to give away this 16-inventory bag and everything in it?!" message.
I didn't think to talk to Live Help. I didn't think there would be anything they could do. Maybe I'm wrong?