A few years ago I sold a rather nice violin through Maple Violins of London (they sell all classical stringed instruments, despite the name). They were a pleasure to deal with. I took my violin to a local valuation session and they made me an offer subject to contract on the spot - I handed over the violin (I'm very trusting!) and was paid about a week later. They clearly had pro buyers on their books that they were sourcing instruments for. I can't remember what commission they took, but I was pleased with the final price. It was more that I think I could have got without a good knowledge of that specialist market. And giving it to a dealer to sell took all the hassle away from me.