Brokers are often a necessary evil, but sometimes they are an unnecessary luxury.
I sold a couple dozen of my best guitars through brokers in 2012. They surely serve a purpose, which explains the 20% commission they get on a sale. They do a good due dilligence, often quite exagerated as they would hate harming their reputaion so, when they do not know and it happens quite too frequently, they do not claim that the guitar they sell is what it is. They have been in business for 10 years or so, and they do have a client database.
Some of these guys are weird. There is one, who is bugging me and sent me a monthly 300$ invoice (12x300$ per year) because I was putting on my website some pictures of MY own guitars which he took in consignment and sold for me. His reasoning is that it is his principles, which he never had me sign on when we agreed to do business together. Its a jungle out there.
Brokers who sell also new instruments will often advantage the new stuff against the vintage instrument you give them on consignment. It happened to me a couple of times.Obviously they have a long term and recurring business relationship with the modern luthier and will present to the potential buyer an alternative. Also, quite often, the broker pretended he could sell my guitars for a certain market price, received the guitar, posted it at the agreed price and then asked me to drastically reduce my price. These guys need vintage instruments on their site to attract buyers, so you are actually financing their marketing cost.
However, brokers are still a solution, as sometimes they do a good job.
For a price...
Honestly giving one of your guitars to a broker is a leap of faith. You will be super happy fifty percent of the time, and pissed off the other half. This is my own experience with them in 2012 with a couple dozen instruments which went through their hands.With Ebay and sites like gypsyguitarfans, brokers do not benefit from the monopoly they used to have. In terms of knowledge, they never share the one they have and hide the ones they lack. I am French and check the european websites and blogs, and players and collectors over there are truly pissed-
We can predict that brokers will have more and more a difficult time selling vintage guitars. They can secure lucrative arrangements with modern luthiers to market their production, but they are indeed offering weak arguments to convince vintage owners to sell their instruments. Vintage guitar lovers must understand that the antiquities collectors are sometimes going to antiquity shops, but they buy directly for a much lesser price from auction sales. If you are buying a vintage guitar you can pay only a fraction from a trustable source which gives you the same garanty. You must however get the insurance you can return the instruments after a one to two weeks trial period, once fully paid.