Dell Latitude E6220 too good to be true?

Good Morning, 

Thanks for taking the time to read this but I suspect a laptop I bought may be stolen, I run my own computer repair business from home as my full time job. A few weeks back a have a random knock at my door just before I was closing up for the day a customer that I hadn't seen before (I've got my address on adverts and business cards)

Anyway he said he had a Dell Latitude e6620 for sale as it was too small for his wife, which sounded pretty understandable. He preceded to boot the laptop into windows 7 professional as I checked the licence key matched the COA sticker, he said he would like a £100 laptop I had in which had a bigger screen and £50 for the dell, It was a good deal so I agreed.

The Laptop sat on my repair pile waiting for me to reinstall windows and remove any old customer data until this morning when I discovered the Bios was setup locked, so I checked the service tag and can see it's still under warranty, Heck I can ever see when it was purchased. I'm unsure what I should do so any help would be appreciated!