Just like any gallery, they will charge a commission for work that they sell. Some sites will ask you for exclusive rights on your work, so that you do not offer the same piece on a competitor's site. If this is a concern for you, you should offer only prints and price consistently. Being part of a cyber art gallery AND having your own personal art web site, will likely produce more results than one or the other. In my opinion , cyber-galleries have some distinct advantages over personal galleries and web pages.
Personal websites are less likely to be found on search engines.
There is more chance of being found in a group. You can be discovered by chance in a cyber-gallery. Someone looking for a certain artist or theme may stumble on you and like what they see. A gallery site may cater to or target certain markets or audiences, which you would benefit by reaching. Site traffic is what generates sales.