For the record; I ship hazmat for a living, (mostly radioactive stuff) I've written about this on this site before. I am what is called a Subject Matter Expert in some circles, and I teach this stuff, but you don't need my powerpoint presentation here.
The only things that all but a very few can ship are loaded ammo and primed brass. There are exemptions for those, otherwise you must first be trained under 49CFR172Subpart H to know what you are doing, (a 40 hour class will teach you enough to make you dangerous) then to ship with the carriers you have to have a hazmat endorsement on your account/contract with them. The training requirement has been there since the early 90s, the contract stuff grew out of ValueJet 592 that crashed in 1996 due to un-declaired hazmat in the cargo bay and killed all 110 people on-board. To a lesser extent 9-11 made things harder too.
Now if you are trying to ship with FedEx you also need to be certified to ship in accordance with the International Air Transport Association (IATA), which is an industry version of the UN Technical Instructions.
The short answer for 99.9% of the people out there is that you can't do it, and the penalties are severe.
There probably should be a sticky somewhere in the "for sale" section to this effect to let folks know.