You won't be able to sell the car with a lien outstanding on it, and whoever the lender is, they're almost certain to have a lien on the car. You would have to pay the car off first and obtain a clear title, then you could sell it.
When you took out the loan, did you not receive a copy of the finance contract? I can't imagine you would have taken on a loan without signing paperwork and receiving your own copy at the time.
If the company you're dealing with is the lender, they are obligated by law to furnish you with a copy of the finance contract (all part of "truth in lending" laws) upon request. It sounds to me like they know they're charging you an illegally high (called "usury") interest rate, and if you have a copy of the contract then you would have proof of it. They'll do everything they can to prevent you from obtaining it, unless you have some help.
I would start by filing a complaint with the Better Business Bureau, because if they want to keep their reputation intact then they'll have to respond to your complaint.
I would also contact the state consumer protection bureau (and/or the attorney general's office) in your state and ask them to look into the matter, and I would see if there are any local consumer watchdogs (local television stations are a good source for this) who can contact the lender on your behalf. Knowing they have so many people looking into this could bring enough pressure for them to give you what you're asking for and be more cooperative with you.
As has been pointed out, keep a good, detailed written record of all your contacts with the lender and, as also pointed out, start limiting your contacts to written letters (certified, return receipt requested) so that you have documentation of your efforts.
Companies like this succeed only because they prey on the fact many people either don't know their rights or are too intimidated to assert them. Don't let these guys bully you, and don't take "no" for an answer until you get what you're after.
Another option might be to talk to a credit union or a bank (if you have decent credit) about taking out a loan with them to pay off the car so you can get this finance company out of your life.