If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
Another key element to the marketing section of your business plan is an outline of your marketing objectives, strategies, and tactics. Writing down the avenues you travel in order to market your business will afford you the opportunity to record what worked and what didn't work. You must be able to measure and calculate the results of your marketing efforts, otherwise, what's the point? If you don't know if something is working for or against you, then it's working against you.
Projected Financial Statements: These statements are usually helpful, but not necessary. You will develop and describe your strategies for the business throughout your Business Plan. In the financial section, you will need to estimate the financial impact of those strategies by developing projected Income Statements, Balance Sheets, and Cash Flow Statements.
The main thing to remember in this section is not to provide new data, but to explain in detail data that has already been provided and to provide the support for that data.