So, in short, a domain broker is an organization that buys and sells domain names. It is, in fact, exactly what it sounds like. Having a good, solid, interesting, and easy to remember domain name is a very important part of web design, and literally anybody can register domain names if they have the money. This has led to some people jumping in and grabbing domain names they think are going to be valuable down the line. They don't do anything with it, they just sit on the domain name and offer to sell it for it's perceived value. Some people have made a lot of money doing this, partially because domain names are relatively cheap to register.
Why Would You Need a Domain Broker
Well, the obvious place to start would be if you had domain names to sell. Especially if you have a high volume of domain names or some domain names that are worth a considerable sum of money, this is where a domain broker would be particularly helpful to you. For example, if you just got your first really big offer on a domain name, don't go all starry-eyed, but get yourself to the broker, who can help you make sure you get what the domain name is worth.
They is also helpful if you're trying to buy a domain name. They will help find and negotiate with the domain name's owner. Now, of course, there's no guarantee, but they can put their time and energy into negotiating and come back to you with offers, instead of you having to do it yourself. It's a lot less stressful and saves you time and energy from having to pursue the domain name you're seeking.
Many domain brokers also offer access to domain auctions, which means even if you haven't been in this world you can jump in and start bidding on domain names you think might appreciate in value. There are all kinds of reasons people sell domain names this way. The owner hasn't sold it yet and just doesn't want to deal with trying to sell this particular domain name, or bought it but is consolidating their domain names down to names they think will sell over a certain amount, some of them are fine with the lower profit but faster work of buying domain names that are interesting and auctioning them off to people who will work harder to sell them for more money. Whatever the case, many of these valuable domain names are being auctioned at a fraction of their value to other dealers and may be a good place to get a jump start into buying and selling domain names.