The term "weapons of mass destruction" (WMD) has a specific meaning. From the DoD Dictionary of Military and Related Terms

Weapons of Mass Destruction — Weapons that are capable of a high order of destruction and/or of being used in such a manner as to destroy large numbers of people. Weapons of mass destruction can be high explosives or nuclear, biological, chemical, and radiological weapons, but exclude the means of transporting or propelling the weapon where such means is a separable and divisible part of the weapon. Also called WMD. See also destruction; special operations.

In general terms, WMDs are either chemical, biological, or nuclear weapons. That's very pertinent to the questions you asked.

Q: Also what country has produced the largest number of weapons of mass destruction?

A: Actually, that was the former Soviet Union. They had more WMD than anyone, including far more ICBMs, huge chemical weapons stockpiles, and a very active biological warfare program involving production and stockpiling large weapons arsenals.

Q: What country has sold the largest amount of WMD's?

A: Off hand, I don't know of any country that has sold WMDs. There's legitimate concern that some WMDs were stolen and illegally sold in the former Soviet states.

Q: What country has used the biggest amount of WMD's?

A: That's hard to say. The US used 2 nuclear warheads to end WWII. However, it was almost certainly the "enlightened" European countries that actually used the greatest number of WMDs in their massive chemical attacks in WWI. The Japanese used chemical and biological weapons in China hundreds of times in WWII, and Saddam used chemical weapons both against the Iranians and the Kurds in the 1980s.