It is not as easy as it sounds; media buying requires some specialized skills to accomplish the task. Primary requirement is the knowledge about your targeted clients. You should also have a sound knowledge about the consumer attitude, the media as well as the parameters that could be the basis of your calculation of various programs.
Salesmen all over will try to induce you in buying a package. There would be many such salesmen who would offer to become your media buying agency. You will better not to be swayed away by any such proposal. Instead it would be better to go for one of those professional and reliable media buying agencies. Once again the bottom line is that you need not blindly believe such agencies. You must have the basic knowledge to assess the proposition at your level without any outside help. Whether you are buying the media for your company or for promoting your own business prospects, such knowledge will help you taking the correct decision.
For example, you need to know what on date the taxation legislation applicable to your case is. Other factors would be media placements, the reach and if it is an electronic media then its frequency dimension, reach and ratings, and above all the negotiated price at which you are buying the media. You can also ask the stations who intend to sell the media to come up with such revised proposals as would suit your needs in order to enable you to select the best media.