I am a Birkin addict and have bought all my Birkins and other Hermes bags at Hermes in London, NY and Paris. Be aware, Hermes bags never come with an authentity card or guarantee card. You will only get a receipt, which is a B5 (or may be smaller) size paper with the Hermes logo - same worldwide. So if a seller is offering an authentity card, I'd get very suspicious.
Use your common sense, you pay at least £4,000 for a brand new Birkin (prices vary depending on types of leather). Would you sell your "never used" Birkin for anything under £2,000????