I was also told (by craftman in NY, my SA and store manager in my store) once sheen is gone, you can NOT do anything about it.
If you want to buy a used bag, and expect that shiny-shine from brand new bag, you will be disappointed.
My cousin bought a used croc. Birkin after seeing my mom's and my new croc. Birkins, she was so disappointed that it was not sparkly shiny...Even used croc. Birkin is still expensive, and of course, she bought under the condition 'all sales are final'. She literally cried.
Can you see the bag in person before you buy it to make a decision?