With the advent of the web, a lot of agents are offering "cut-rate" commissions. Most of the time, lower commissions are tied to a lower level of service. If all you want is to be listed with the Multiple Listing Service and a sign in the front yard, then a cut-rate commission may be right for you. If you want an agent who will actively promote your property to other agents and spend money on advertising, then you probably are not going to get that level of service with a reduced commission.
At other times, the lower commissions are offered when you agree to tie in to other services offered by the broker, such as agreeing to use a specific lender, escrow, settlement, or title company. The broker (not the agent) will probably have some type of ownership or profit participation in those businesses. The problem with agreeing to tie in to these other companies is that they do not have to be as competitive in pricing their products or services.
Another common practice when you see an ad for a reduced commission is that the compensation is lowered when you agree to buy your next home through the same agent or broker. Usually, the reduced commission is not really being offered on the sale of your existing home but on the purchase of your next one. The ads are usually unclear on this.
As a result, when you see an offer for a lower commission, you should analyze what you are giving up by accepting such an offer. It probably will not be readily apparent in the advertisement. Be sure to ask lots of questions.