I do not have any fund and have a job, but I was thinking about starting something aside of my job and try to make it known. The biggest problem for me now is n

I do not have any fund and have a job, but I was thinking about starting something aside of my job and try to make it known. The biggest problem for me now is not technical, I could write an app to develop my idea and put it online. The problem is that I don't know what I am allowed to do or not allowed to do as an owner of a website/webapp that I put for free online. If you take for example the case of Craiglist, someone will put an ad to sell something, and potential buyers will contact this person. How does the website discharge itself from being responsible from any failure in the process? Every law related question are pretty hard to find an answer to.