As a general rule, you should not engage with door to door salesmen. If you are interested in the product they are selling, check online first.
Selling Magazines
Someone or a group will come to your door and offer to sell a magazine subscription. Often the subscriptions are not for the duration or price you were told, and the magazines will often have tough or impossible cancellation policies.
Energy Sales
Somebody will come to your door claiming to be from an energy company. They will ask to see your current energy bill so that they can see how much you pay. They will then offer you a discount if you sign up with them, and promise to handle everything with your old provider. Some of these scammers will "slam" you, by using your account number that they saw on your bill to switch you to their service without authorization, and some will scam you by charging higher prices than the ones you agreed on.
Security System Scams
Scammers will come to your door and ask about your security system, and offer to sell you a new one. These scammers are either selling you overpriced low quality products, or are casing your home for a future burglary.
They ask to enter your home
While trying to sell you whatever, they suddenly need to use your bathroom, or they've been writing against the wall and ask to use your table instead. Or maybe they just moved into the neighborhood and want to see how you decorate for ideas.
They're scoping out you and your place. They want to see what valuables you have, how gullible you are, if you have a security system or dogs, etc.
Miscellaneous Scams
Money Flipping
Scammer claims to be a banking insider who can double/triple/bazoople any amount of money you send them, with no consequences of any kind. Obviously, the money disappears into their wallet the moment you send it.