Its tempting, you look at the price of software from a well known vendor and its less than half price on eBay. If this is the case there is a good chance the software is not legitimate. In plain English this means someone is selling you an illegal (stolen) copy. In my years of working for software creators I can tell you the margin the resellers make is not that generous, so if the discount appears to be good, its probably too good.
How do they do it? Well they either use software which they have modified meaning your updates are not always available for bug fixes, or they may even have put spyware in there. Another scam is to use a corporate key. Software vendors like Microsoft give large clients a key that works everywhere, so when you get the key it will work, but once the vendor works out this key has been resold they will ban it meaning you have to purchase another copy.
So you get a license key what is the problem? Would you buy a 50 inch flat screen TV brand new for £100 because someone tells you they bought them in bulk? In reality you know its probably stolen! The judge won’t bother listening to your excuses, and if you sell your business and the new owners find out, I have seen instances where they charge you the cost of buying legitimate licenses.
Buy your software from a traceable source, a certified reseller of the software. If there is a problem they will resolve it, or the software vendor will work with you to find a resolution.