Forums are good place for selling products and services as well what you need to do mainly is to contribute to the forums first, if you don't contribute to the forums then you may not be active on that and hence no selling, no learning ..........

To sell your services you have to start by making your posts interesting and attractive for someone to read. This will firstly demonstrate that your product might be good as was your post, secondly by reading the entire post, user will get to the spot where your link to the products is located. Otherwise, no sale.