Did I commit copyright infringement?

I am a college student and I am in my senior year. I have just received a letter explaining that I am being sued for copyright infringement. About a month ago I was searching for the solutions on the internet for one of my classes textbooks. I found many places (eBay,Amazon,many personal sites) selling the file versions of the solutions for my book. I also found a site that had the solutions listed for free which is where I downloaded it.

After seeing that eBay and Amazon had the same thing listed, that I now owned for free, I decided to put my file up for sell on these sites. I knew eBay had strict rules so I was not worried about putting it on the site because other people had them listed. About a week after I list the file it is taken off eBay and eBay says that they were contacted by the company that owned the copyright to the file and it was removed for copyright reasons. Now I have this letter which explains what damages they can recover for each file I sold "not less than $750 or more than $30,000 as the court considers just". The letter goes into more detail and gives info of what is going on and it also has acceptable resolutions listed: basically it says that they are willing to settle early without a lawsuit if I: immediately cease selling(I did); provide all info I have on others who may be distributing; provide records of parties I sold to, and if I do it in the future it will be $5000 per copy. They also require a payment as compensation for the infringement and they want me to contact them within 10 business days or I will be acting in bad faith. I sold around 25 copies of the file for about $5-$7/file.

I also want to say that I did not know this was copyright infringement and that I went through the entire file and it does not have the companies name or any copyrights. Please tell me what I need to do before I go crazy worrying about this. Do I need to find a lawyer before I contact them? What information do I need before contacting them? How much are they wanting to sue for?

Often they will not actually sue due to costs etc. if you take down all sales posts and do not do it again. You could just wait to see what happens or have a lawyer intervene. See the adidas copyright posts on this site dealing with the same issues basically.