Impulse Buying, as far as reptiles/amphibians go, is when someone purchases an animal without first preparing and/or researching. Often times they will be browsing a vendor's table and a certain critter will catch their eye and before you know it money is exchanged and you're holding a living, breathing animal inside a plastic travel container.
While it's easy to get swept up by an animal's adorableness, it is often unfair to bring them home on a whim as you don't have their enclosure ready, food on hand, or possibly even know the basics of their care.
This is why, if you're planning to attend an expo, it is better to do your research beforehand to see what species you may be interested in and try to get some background on the vendors who will be there. Also, make sure the enclosure is ready before you go to an expo so your new friend can get settled in after what will be a nerve racking day/days for them.