If you are doubting its authenticity, get it authenticated.
On a side note, unless you need the money, I would suggest that you keep it.
Burberry bags depreciate in value quite a lot unfortunately. If the standard is just a 15-25% drop in price, I would expect to start off at least a 30-50% drop for Burberry. They just don't hold value well.
You can search around on
Fashionphile or
TRR to get an idea.
When you bring it to consignment, it may be a bit of a shock to you at the price they may offer.
Good luck!