Are you sure? Sometimes its hard to tell! [/joke]
One thing I would suggest looking into first before going the software swap route is to double check the EULA agreements for the packages you intend to purchase this way. Many software makers have purposefully crippled second hand markets like this one, by forcing you to agree to never exchange the registered key with another person, ever. This makes it impossible to legally obtain support services from second hand software that has been so buggared by the developer OEM.
Depending on your country of origin, such tactics in the EULA may or may not be legally enforcable or applicable.
(Some noteworthy software titles KNOWN to be this way are 3D studio Max, MasterCam(don't ask-- part of the EULA is to not disclose the eula! For real! It's that onerous. Since I didn't agree to the EULA, I can discuss it all I want!
) Microsoft Windows and office products (to sell a copy of windows, you have to obey their system builders rules. They are quite restrictive. Alternatively you have to be a retailer, and sell at retail prices, using product straight from microsoft. There are of course, others. The enforcability of EULAs is a contested matter. I just want you to make an informed choice.)
You need to make an informed choice, much like choosing to root a phone or tablet, or install a custom firmware on a router. Your choice should be with certain considerations in mind, and should not be taken without those considerations being....well.. considered.
On a personal note, I think such EULAs are damnable things that should be illegal, as they violate the principle of first sale. My opinion and what the laws in your jurisdiction are, however, are not the same thing. Self-educate, act accordingly.