Beware buying used software... if it requires online validation, the company may not allow you to do so unless you're the original owner. Contact them if you're unsure.
I know Autodesk certainly works this way with their software, not sure about Pixologic.
Try the 30 day demo for 3D Coat. Before I owned it, I used it for several months by just downloading the new release of the demo each time it was updated...which was certainly more often than every 30 days, thereby giving me a lot more time to consider the product.
3D Coat is also cheaper than Zbrush by 1/2 and has a student discount which makes it even more affordable.
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they may have student prices if you qualify.
I know Autodesk certainly works this way with their software, not sure about Pixologic.
Edit... there are less expensive alternatives for digital sculpting... http://wiki.polycount.net/Digital_Sculpting
3D Coat is also cheaper than Zbrush by 1/2 and has a student discount which makes it even more affordable.
www.3dcoat.com