Do I need to put all the ingredients on the barbecue sauce that I want to sell online?

Federal and state laws require that most of the ingredients be listed on the container. However, tiny amounts of certain things like spices don't have to be listed. Check with the FDA, the USDA, and your state for specific requirements.

I don't know for sure - but my best guess is yes - due to potential allergy situations. Having a secret ingredient is an excellent concept (trade secret) but may cause problems in food products. You might be able to list certain ingredients generically - spices, salt, etc. If the mixture is complex - it is unlikely that someone could duplicate it. Think COCA COLA - never copied exactly.

You need to follow the FDA guidelines and any others that apply. There is usually some room to list "spices" for things that do not have to be independently listed. Start researching here: http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryInformation/LabelingNutrition/ucm064880.htm

I believe that to meet FDA requirements all foods must be labeled with all ingredients in descending order of their amounts. However i am not knowledgable of these type of laws so please check with the FDA for the labeling requirements. It is outside of the scope of my legal expertise. I am a Patent and Trademark Attorney.