First off I must say the service from this web page alternates between neglect and intense focus. I was able to buy tickets to Arsenal v Sunderland, and the tickets were amazing but until I got the electronic version I was on edge wondering when I would get them. If you order more than a week in advance you should be fine. They do what they can, given how difficult it can be to obtain tickets and the prices are relatively fair. As a note prices can fluctuate with time, so keep that in mind when purchasing.
The tickets are legitimate, any issues that could exist are a result of the flexibility of football scheduling and policies that the website could not foresee.
I would wish that they give you the option of reselling the tickets to them so that you could cancel your order and have the possibility of a refund.