is it legal to buy a replica rolex? online or via craigslist?
No - You should only purchase authentic products. Some governmental agencies arrest and/or fine people who are wearing or using replica products.
Personal Injury Lawyer
The distribution of counterfeit brand name watches is considered a form of trademark infringement. The practice is subject to potential civil action and criminal charges. In many jurisdictions, there are also laws against purchasing counterfeit items. You should only purchase authentic items from authorized sources.
Patent Application Attorney
So in my opinion, if you are in the US, and the "fake" item you are referring to is a fashion item subject only to possible copyright or trademark infringement, but not patent infringement, (for example, a watch having the Rolex logo that is not actually a Rolex), you can purchase that item for private use and you will not be breaking any laws or incurring any civil liability to the trademark or copyright owner.
That doesn't mean you should do it. Why is it important to you to have a fake Rolex? Why not just buy an authentic watch of a different brand? To the extent that a Rolex is of superior quality to other watches, the fake watch certainly won't have that quality. And so all you are paying for is the word "Rolex" in hopes of impressing someone who cares about such things. On top of that, you are helping someone who actually has broken a law by making the infringing good in the first place. Counterfeit goods distributors are often linked to other kinds of illegal activities, such as terrorism, drug trafficking, theft, forced labor, poaching, etc.
And if the idea of a "real" watch is too boring for you, consider spending some money on a watch created by an artist or a new start-up company. Something like a Pebble Watch (see link below) or one of the cool handmade designs available on Etsy (see below). Handmade real is way cooler than Chinese-made fake.