Do you own the software you legitimately bought?
It appears that some companies believe you don't and that they are selling you what in effect is a licence which may not be resold. In the USA a judge has recently rejected such a claim in a case brought by Timothy Vernor in response to eBay acceding to a request by Autodesk that eBay stop such sales. This case has been widely reported on the web in recent days. Here
is the ITWire version.
It will be interesting to see how far this judgement affects the software market. The precedent apparently concerned "legacy media" and one significant difference with software is the ability to obtain version upgrades at reduced prices. The same option doesn't occur with, say, books where it would be perfectly legitimate to sell on part one of a trilogy. Obviously selling on v1 of a software package once it has been used to install a reduced price v2 upgrade wouldn't fly morally and it would presumably also be illegal.
But if you could sell on the previous full version of your favourite photo-editing software at a price that allows you to buy the latest full version for less (net) than the price of an upgrade we might see a healthy market develop. This would be a nightmare if the vendor forgot to "de-activate" such software before uninstalling it, though. I fell foul of this activation issue simply by transferring software from an old to a replacement computer as I reported here