We recently did that. We purchased the upgrades, installed them and then gave the customer all the original discs and the new upgrades. We didn't keep anything. You can't even sell a used computer, technically, if you don't have the original 'Windows' disc to accompany the unit.
Its more headaches than its worth in my opinion. Yeah, you might be saving some money, but if you don't get all the documentation and get everything transferred into your name and get all the key codes and dongles your used software you just got a good deal on is WORTHLESS because you won't be able to use it without all that! The difficult part though is if you do buy used software you have to trust whoever you're buying it from that they are giving you everything you need to use the software, but how are you to really know for sure because you didn't see what was originally in the box when it was bought brand new you know what I mean!
Is there one particular software you want answers on?
I have some valuable experience with the answers to this, related to several different software, but I'm not into typing out all the different answers right now.
Generally I would disagree with the above statement. Obviously if you get ripped off, then it's a bad idea, however if you get someone from here who has the real deal to part with... the steps to make it legal are minimal IMO. (for the software I know about... Adobe, or flexi, or Estimate... etc)
Over the years I've bought a few used legit Flexi-pro with dongle and later upgraded them. It's been great for us, just realize you must register under your name and pay a transfer fee which could be rather expensive. Every software company is different so do your homework on the particular software you are contemplating to make sure it is legit.
The short answer is it depends. But, yes, most software companies will allow you to do it. Some will require you to jump though hoops and/or pay transfer fees while others are very easy and don't charge you a fee.