I stupidly signed a contract for new exterior doors for my home without researching first. The contract was for 3 new doors. The day after I signed the contract, I tried to contact the salesperson, he did not return several phone calls and e-mails were returned (not a valid address). They contacted me after the 72 hours and stated that I could not cancel and they would charge me a 50% restocking fee. The contact was for approximately 10K, of which I paid 50%. I decided not to cancel and loose my deposit and had the doors installed.

I researched other local companies that sell the exact doors and received a quote for a little less than 5K installed. When the owner of the company came to my house to demand payment I refused and showed him the pricelist for the doors. He now is threatening to place a lien on my home for final payment. He said I signed valid contract and whether he overcharged or not is irrelevant.

I wish we could read the entire contract but it sounds like you did your research after you signed it. I am cinfuesd by your statement "stated that I could not cancel and they would charge me a 50% restocking fee" do you mean you'd be charged 50% if you canceled or what? If he goes to court you will lose.

I'm sorry to say that he has a right to file a contractor's lien.
Your only hope is to investigate his license number on your state website (you have it, right?) to see if he is current and to see if there are complaints filed with the Consumer Protection Div of the AG's office. You don't have a valid complaint yourself, but you might be able to scare him away if he has a poor record or let his license lapse.

Ok.. what happened was this vendor was at a local fair and I gave them my contact info. In Nov. a salesman called me and said he was in the area and would like to meet. At that time I was taking medication and under a physician's care because of a family death. My judgement was impaired as I would never sign a contract for such a large amount unless I researched the company and had comparable estimates. Within 24hours clarity returned and I tried to cancel the contract within the 72 hour state statute, I was not able to speak with anyone within 72 hours. The contract clause is a 50% fee of total contract for cancelling. Fully cognizant that I was dealing with an unscrupulous company, I decided to let them install the doors rather than lose the 50% deposit. In the meantime I asked for a quote from a similar local contractor and they sent me the customer price list of the exact doors and installation fee. The total was slightly less than my down payment. I can prove that the charges were most likely usury. There are no complaints on the license. Although they operate under from what I can tell 2 different names, same address, different CEO's.

I'm sorry, but your medication and state of mind aren't going to help your case. I suppose there is a lawyer out there somewhere who might try for you, but I doubt it.
Usury refers to interest rates, not quotes for work.
You are free to sue, of course, such as in small claims (the max according to locale 3500 - 5000) but I suspect you will lose, and lose the filing fee.

If he does place a lien on your house, that means it's on record at city hall the same way a mortgage or overdue tax bill is. Nothing happens until you sell the house or die.
If he decides to sue in small claims instead, respond to the suit regardless of how you feel about avoiding it, because failure to respond will mean he wins by default.
If he wins, he then has to proceed through other suits until finally a bench warrant is issued (which is nothing like the search or arrest warrants you see on cop shows). Check the details of this with your city hall.

Where do you live? I believe the 72 hour rule applies to door to door type salesmen, one who approach you cold. You approached them and gave them your information at a fair so that rules does npot apply but tell us where you are located.