It's only when they are inside that you find out that they want to sell something! Ask for some kind of identification and don't let anyone in until you know who they represent.
Some salesmen may try to insist that you make a quick decision to sign a legally binding contract. They may mislead you into thinking you can always cancel and back out later when in fact you can't. You may be offered a discount to tempt you to sign on the spot without checking on what other firms would charge.
Be on your guard! Don't feel obliged to buy anything you don't really want. If you are interested, get written details and a price from the salesman and compare it with other firms' estimates. And make sure you get the name and address of the salesman's head office in writing - certainly before you part with any money.