I contacted them three times (through different means over the space of a month or so) when I was looking to sell my turquoise Birkin. Never did get a reply!
I still think Xupes takes some beating when it comes to customer service, they were great when I've bought or sold through them. Prices are high if you're buying though but I do trust them 100%.
Designer Exchange have been great for buying, however a very low offer when I've contacted them to sell something.
Naughtipidgin's nest
https://www.npnbags.co.uk has probably the loveliest owner you could ever hope to deal with! Again totally trustworthy for purchases or selling. Commission is very reasonable for selling (only 10% for items over £3001) however the H selection is generally quite small and she closes (or used to) for school holidays so I think some things may hang around a little longer depending on the time of year and who spots them.