It's a lovely piece! Production cels (the ones filmed to make movies) are inked & painted on a clear film-like material. It looks like what you have is a limited edition piece of art expressly created for public sale. A scene such as this on an actual production cel would be worth thousands of dollars.
That said, I'm not an expert, just a fan who has a couple of production cels along with some ltd edition pieces that look similar to cels, but were created for sale (at a much more modest price.) Did the person who sold it to you have any info at all?
BTW--Welcome to the Dis!