Well the main difference is cost. A roller setup will cost you about $500 and go up from there ($250 for cam and $250 for lifters) that is for a fairly cheap set-up not top of the line.
A flat tappet cam and lifters can be had for $150 (and that is for good quality, name brand stuff).
If he charged you for a roller and installed a flat tappet he owes you quite a refund (IMO). Before you ever had the engine built you really should have done some homework, while I do not think it is your fault by any means. The problem comes in when people don't know what they are talking about or have any knowledge of the service they are asking for. An honest shop would have asked you all kinds of questions and then made suggestions to you. It is not hard to talk to someone and figure out if they know their stuff or not. Crooks will figure out you don't really know engines and then over charge you for service they never actually did (or in this case products you didn't recieve). If you have it in writing on the reciept then you have him by the short hairs.
How do you know it is not a roller cam? Did you look up the part number? Just trying to help you figure out if you were ripped off or not. Did he maybe tell you he was going to put in "roller rockers" and you misunderstood that he meant cam?