If you are planning to buy from a private seller, make sure to see the machine and check if it is in a good working condition. Test the machine before deciding to buy, do not just rely on a visual inspection. While the machine is running, observe for any strange noises, check for funny smell and other behavior that would indicate that the machine may be damaged. Ask the seller why the machine is for sale and if possible ask for repair documentation and warranty information.
If buying from an appliance store or from a used washer and dryer dealer, be sure to check for the store or dealer's reputation. You will be able to check and find a reputable and trusted seller based on customer reviews available online or you can ask your friends or family who have previously bought from the store or the dealer. Ask the store or dealer for their terms and conditions as well as their warranty and delivery options.
It is always good to ask questions, so don't hesitate. A wise and informed buyer will always grab the best deal in town.