If I remember correctly, I think he did a huge number of warehouse signings for TEA that were then sent out to the booksellers. However, this would have been 3 years ago and it's most likely that all of those books were sold.
Do you know if it's a first edition? If it's first edition, it was probably the warehouse signing. If it's not a first edition, and that particular Barnes and Noble was on the list of venues for his book tour in the last 2 years, it could have been a store stock copy that he signed.
In fact, if you'd like to read more about his signatures, you can check out this section about
signature styles And here's a section all about the
US Hardcover of TEA. That covers the first and second edition. I can't remember if it has hit 3rd of 4th printing yet.
Hope that helps you figure it out!