I saw a website which was selling things, but a few of the items were $0.00. If I was to buy the item, I'd only have to pay shipping for it. I'm no expert but I've been around the block and the website seems legitimate and it just looks to be an error or mistake on the website. Would it be illegal to take advantage of this and get the item for free (not including shipping)?
If they actually mean $0, then that is not "taking advantage". If they do not mean $0, it is most likely that they will tell you "Sorry, we made a mistake, we're not gonna send you that Rolex for $0 plus shipping". If this came with free shipping, then you would not actually have a contract, because there s no consideration on your part (no payola). Fortunately, there is shipping, so there is a contract. You could then attempt to force them to send you the goods, which they might do rather than irritate you, but not if it is a Rolex. One of the defenses against enforcing a contract is "mistake", and a $0 Rolex would be a great example of that.
Things get a bit more tricky if you relied on their free Rolex. You would look up the doctrine of promissory estoppel, to see if the seller could be estopped from making the mistake argument. Let's say that you also bought a Rolex Display Case from someone else at a cost of $100 plus shipping. By relying on their promise to sent you a Rolex, you have suffered a loss. The most likely outcome is that they'd have to reimburse your Display Case expense.
(Finding) mistake airfares is an industry: a common response for the airline is to say "Oops, sorry", though sometimes they honor the mistake fare. Rumor has it that rather than get trashed on Twitter, the airlines honor mistake fares. You may find disclaimer language pertaining to verification of prices and availability, which also gets them off the hook.
At any rate, you certainly won't be sued or prosecuted for assuming that they mean it and buying the thing; you might be disappointed.
Selling for $0.00 is a advertising gimmick. The seller makes their profit by inflating the shipping (and usually adding handling fees). Years ago before I moved to ebook format, it was not unusual to find used books on ebay for $0.00 or a penny. With shipping being 2 or 3 times standard rates. You see the same thing on TV all the time, "Second Unit Free" just pay extra shipping and handling.
Use extreme caution if the shipping and handling fees are not clearly identified. If you agree to an undisclosed amount of shipping and handling fees, you are obligated to pay them. As shipping and handling are seldom covered in low budget returns, the seller will only be obligated to return your actual purchase price.
In the end the only one being taken advantage of is the buyer.