A Gold Mower
When someone tries to sell something online, they will generally specify if it is for sale, if it is for free, or if there is some sort of another arrangement that can be agreed upon. But if nothing is specified apart from the price, then it is safe to assume you are meant to pay for it.
But every once in a while you might come across someone like this that thinks it is appropriate to try and trade you a lawnmower for a $19,000 car. The seller, stunned, asks this person if the mower is made of gold, but he responds “No it’s red”. It is best not to interact with these people at all.