Zev,
Assuming you’re buying in the New York area, I can give you an idea how it works here. (Just to be clear, let me give my cover my butt speech: I am not your lawyer; this is general information not specific legal advice; if you really want to know what’s what, talk to your own lawyer or other professional, not some random yahoo posting on a message board, etc.)
In New York City and the surrounding counties, the broker’s commission is paid entirely by the seller. Typically the total commission is 6%, but this can be changed by agreement between the seller and the broker(s).
Typically, the seller will enter into an “exclusive” agreement with a broker to list the property. The listing broker will then place ads and do everything else necessary to list the house.
Most commonly, buyers will be assisted by a broker in their search. If the buyer has a broker, and a deal is reached, the two brokers will split the commission (50/50 in New York City).
The listing broker for the property expects this split, and if there is no broker to split the commission, it is a major bonus for the listing broker. Sometimes where a buyer comes in with no broker, the buyer can negotiate to lower the purchase price a few percent and take it out of the broker’s “extra” commission, but this isn’t all that common.
(The above relates to traditional broker’s fee schedules, and not “discount” services like Yhd Foxtons, which I don’t really understand, but I believe that will increase the fee to the seller if the buyer comes in with a broker.)
In short, there is really little down-side for you to use a broker a buyer in New York, and having the market information that a broker can give you can be very helpful.
E-mail me if you have more specific questions.