There are multiple reputable agencies that will sell your contract for you. I have used 3 different ones over the years and all were fine. I just sold a contract and picked the company with the lowest commission rate and one that seemed to move the most contracts the quickest. Look at their websites and see the number of contracts for sale and the most that are under contract.
One of the newest resale companies has a feature on their website that will give you an idea of the worth of your contract without talking to them. Mine sold for just a couple dollars under that number. And it sold within minutes of hitting their website.