Although a few cases are winding their way through the courts that may change the law, at the moment software used by consumers is licensed and not purchased. Which means that the "owner" of the software only owns a license to use the software and so, typically, he or she does not have the right to sell or otherwise dispose of it (including by sublicense). The terms of the license control how the software may be used and transferred. The licenses that come with the vast majority of software used by consumers does not permit the owner of the license (the "purchaser" of the software) to re-sell the software or "transfer" the license to anyone else.
I very seriously doubt that the "software shops online" that are offering reduced price software for sale are doing so lawfully. Otherwise, there would be a thriving "used software" marketplace -- which is certainly not the case. If you buy software from someone who does not have the right to sell it to you that purchase is an infringement of the copyright in the software owned by the software's manufacturer. Software manufacturers very vigorously enforce their copyrights.
It is a well known fact that there is a cottage industry of parties buying off the shelf software, making illegal copies of that software, and then selling such software in the marketplace. By and large these are unauthorized resellers, who are essentially trafficking in copyright infringing wares. As mentioned in the other post, the EULA is typically phrased as a one person/one machine license that does not permit resale, or, at the very least, reproduction. Hence, "caveat emptor" is the operative doctrine here. Ultimately, in the case of the innocent bona fide purchaser, as a practical matter, little will come of it other than having to purchase a bona finde copy of the software. However, in the business context, it can be very damaging to be caught with illegal software. And I routinely advise small businesses to carefully evaluate the source of their software for legitimacy.
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.