If I sell merchandise, what kind of mark-up do I need to profitably market these products on the Internet?

The answer to this question is contextual to the type of organisation/brand you have and the type of marketing campaigns you plan to use.

A simple way to plan for1 quarter is as follows :

1. Use your current sales to forecast next4 months sales.

2. Now sketch the expense of marketing campaign you plan to launch for the next4 months.

Marketing expense/ foretasted sale = extra expense per $ earned (Lets say X)

Add this amount as an extra markup.

Ex:

Sale forecast for4 months : $1000

Marketing campaign expense : $200

Extra expense per $ earned =200/1000 = $0.2

Cost of my SKU =10

Additional markup =0.2 x10 = $2

New Selling Price = $12

Hope this helps!