I haven't been a member of tPF forum for long, but I know you can sell them through the Marketplace on this very site!
Also, if you're in London, there's a store called Bang Bang (well, two stores: one in Soho and one on Goodge Street) that will buy your vintage bags. They tend to have some really beautiful things and they know their stuff. The great thing about them is that if you're looking to buy as well as sell, they'll either give you cash or a higher amount in store credit. I've sold some Megan Park pieces to them.