I'm only addressing the software issue here. I am a dealer so I am personally bound to respect the licensing agreements but I realize people are going to do what they're going to do in secondary markets. To some degree it's Radiant's own fault that people do seek out these third party licenses. The vast majority of dealers are very reluctant to sell software w/o hardware, and also reluctant to sell anything but their own Radiant hardware, which customers may not want. In the cases where dealers would sell software only, they effectively doubled the price to gouge the client.
In short, you can set it up and use it, but not legally. The practical reality is that not much will be said or done to stop you in the cases I've seen, but if you ever need the dealer for anything-an upgrade, support, a patch, etc- the dealer can refuse to service you until you pay a transfer fee, and possibly an upgrade fee if it's a version they don't support anymore. Some dealers are more cooperative than others.
I think I even heard of one case where Restaurant A out of Florida closed it's doors and sold the software to Restaurant B in South Carolina on Ebay. When Restaurant B needed support and/or went to make the software key "legit" via the local dealer, it was discovered that Restaurant A (now closed) owed the Florida dealer a balance and was told they'd have to pay that too if they wanted any type of service.(the names and locations have been changed to protect the innocent) An extreme case no doubt, but still a scenario you should be aware of.
The other issues are: How do you know what that the security key is licensed for what the person your buying it from says it is? (And I am not just talking about someone lying, some people legitimately just don't understand what it is they originally purchased) Also, make sure it's at least a PCI compliant version-I've had client buy them on secondary markets, only to find out they were version 4.x and pretty much useless for their needs.
It's up to you to weigh the risks and decide if it's worth it. My only advice is because of the risk, if you do it, make sure it's a smoking hot deal-like 60-80% off what they key would go for if you purchased it new. If it's just a "good" deal or "ok", I personally don't think it's worth the hassle. In that case, find a dealer who will sell you just the software at a reasonable price, and you won't be left worrying about it.
Hope that helped.