Generally these kinds of things are either:
1 - Someone has downloaded a pirate version of the software and is selling it. the licence key will either be auto generated or from a batch that the pirates have.
2 - Someone has bought and downloaded one copy of the software from the manufacturer legitimately and is now selling multiple copies of it to recoop the costs. The licence key will be legitimate, but when you try to register it you will find that it has already been registered.
In either case you will not have a legitimate copy of the software and you will probably find that the next time it tries to do an automatic update it may well stop working. You certainly will not be able to use it in order to qualify for the upgrade prices to the next version.