What to check out and think about pre-purchase?
Here you can try out the full version of Smart Slider. You can also download and install the free version to see if it's working on your website, as you would expect. After that check out the demos, where we used almost every option you can use with the Smart Slider.
You also have to decide which package to buy. What you should really consider before purchasing is that on how many sites would you like to use Smart Slider. Not because of the number of websites where you can install the slider, because that is unlimited, but because you can only request support, turn on the update feature, and turn on the demo slider downloading feature at as many websites as many licenses you have.
Also check out the other questions here.