I will buy your horse, car, guitar, motorcycle, car etc., etc., etc. They will tell you that they are interested in buying whatever you have for sale. Somebody in "your" country owes them money and they will have that person/company send you the money in the form of a cashiers check.
The check will be for more than your asking price for your merchandise. You are requested to return the "excess" money via Western Union. They may tell you that it takes only 24 hours for a cashiers check to clear! It does not! Being an honest citizen, you will send (return) the overpaid amount. Later, much later, you will find out that the cashiers check was fraudulent and now you owe your bank that money.

Here is an actual email from a scammer to a potential victim:

hi ,
  i have just seen your Mercedes-Benz 300E car advert.and  i'm very much interested in buying it,i will also need pictures of the car.i'm based in cotonou ports fedral republic of benin,west africa.and i will be making the payment buy the use of an
american bank certified cashier's check,there is this US based collegue of mine who is   owing me some funds $13150 and he is now ready to pay me,so if you will be ready to sell the car for me,i will make it$6150 to secure the car from selling it to another
person.the check will be adressed in your name, so after the funds is cleared.you will send the excess funds to me viawetern union money tranfer. because that;s the only effective money sender effective in this area.i will personally arrange for the shipping of the car.pls kindly send  your fullname,address and phone number.so i will send the data to my friend and the check  will be adressed  in your own name and it will be sent to you via FEDEX.pls i am looking forward to hear from.thanks.
aminu