Merchants Credit Guide ripoff Chicago Illinois

I recieved a phone call yesterday that left no name of the company, when I called back I was told that I owe $1800 for a 10 year old Sears account, I told him that I would need verification of a debt that is 10 years old and that by law in Missouri the statute of limitations had passed, at that he told me that Pinnacle Credit would be reporting this to the credit reporting agencies in 2 weeks and that I needed to pay my bill and not threaten him, then he hung up on me.

I called back and spoke to a woman who told me I would be recieving something in the mail in the next couple of days, I recieved a bill today telling me that Merchants Credit Guide had purchased this account from Pinnacle Credit Services and they want their money.

I dont know what to do now, I will write them asking them for verification of the debt but in that do they have to prove when the bill was turned over, I know this is just wrong becuase I know this debt is way to old but him threating me with putting it on my cedit report really bothered me. Any advise is greatly appreciated.
Thanks

Pam
St.Charles, Missouri
U.S.A.