This is the site we use for vintage: http://vintagefashionguild.org/label-resource/gucci/
There is a great listing from A-Z.
Be aware there is also a trend of people taken labels from trashed luxury designer items and sewing them into other items to fake. Ask the store what proof they have for authenticity. They should be able to explain and show using the web. You can also join the Vintage Forum and have them authenticate it. Anything we posted takes about 24 hours to get input from industry insiders.
Just a tip - they do not give out pricing or quotes ever! Will get their panties in a bunch to even hypothetically give what they think the item is worth. V.