Another type of scam that has recently been brought to my attention is one when dealing with those you are selling to (like from kijiji.ca or kijiji.com, craigslist...etc). You'll find a lot of scams on their as well as people wanting to scam you when they buy from you.
First off, don't let them tug at your heart strings. Some of these people will tell you a sob story to make you feel sorry for them. Often check out their other ads (a lot of them do) and you may find that they are asking for free LCD TVs or other such stuff that really doesn't coincide with their dilemma they are spouting to you. If you do feel sorry for someone or think maybe their is some truth to what they say, simple advise them of avenues that can help them (Goodfellows, Goodwill, numerous charities...etc). Trust me, these organizations are better equipped to offer the right type of help and guidance.
Second, when selling stuff, always make sure the buyer sees the product as it should be (it works, no holes, clean,...etc). This way they can't later claim that when they got home and tried, it didn't work, had holes, or was defective. Yes, it's always "Buyer Beware" but you should also have "Seller Beware".