You might have come across local selling pages and Facebook groups that are focusing on one particular brand, are really the next level of this. In this case her they are a home for Mulberry-
As with all pre-loved purchases, it is up to you to make sure you buy the genuine article, i.e. ask for additional photos and get the bag authenticated if you feel unsure. However, the sites I am listing below are moderated by some lovely admins who keep an eye out for anything that shouldn't be there and members purposely listing fakes are banned. They also tend to stipulate that you pay via PayPal, which gives you buyer's protection, should you receive an item that's not genuine. Lastly, these pages offer space to ask questions, lust over other members' collections and reveal your latest purchase.
Mulberry Buy and Sell (formerly Love Mulberry? Buy, Sell and pass it on)
Mulberry - New & Preloved - Buy, Swap & Sell
Louis Vuitton, Chanel, Celine, Hermes etc., Buy and Sell
There are also some Facebook pages from individual sellers, the following just being some examples that I am aware of. Check the individual's Ts & Cs in re. to returns etc and double check authenticity for your own piece of mind.
Aqila's Facebook shop. A great place to find her lovely bags and accessories for 11% less than on her eBay shop (avoiding the eBay fees) and to have a chat with the lady herself. Just pop her a message if you're looking for that elusive bag or have any questions. Don't forget that she now also has her stand-alone online shop Papillonkia.
Addicted To Handbags
Run by Jade, you will find great pre-loved Mulberry at very reasonably prices. Fab if you're on a budget.