If you are selling things to make some extra money, then you will want to get the best price for them that you possibly can. This could be things that you have owned that you no longer use or items that you have bought to sell on, both are equally important.
Firstly, you will need to make sure that the item looks good. This may mean that it needs a wash or other sort of clean. Obviously the cleaning that you do will need to be appropriate for the item and what it is made of. Do not just clean something without checking the best way to do this. For example do not wash dirt off an oil painting, but just gently dust it and do not use harsh cleaning products on wooden furniture but dust it and use wax. It is wise to at least search on the Internet to find out how to clean things, but if it is really valuable you may want to consult an expert.
If you are selling online, you will need to take photographs of the item. Take them in a well-lit room with natural light, so that you do not get reflections or fading effects from the sun or artificial lights. Take from different angles and take some close ups as well as one from further away. If the item has any damage photograph that as well. It is much better to show damage so that the potential customer knows what they are getting and have no reason to complain or ask for a refund.
If you are describing the item give as much information as you can. Explain what it looks like, what functions it has, what the condition is like and give details of damage. This will allow customers to know as much as possible and by describing things in detail they will be more likely to trust that you are being honest with them and will be more likely to buy.
Giving customers lots of options of how to pay for the item as well as how to have it delivered or collect it will mean that you are more likely to get a buyer. They will want to see that you are willing to accommodate them, so allowing them to collect it from you, to save on postage or offering to post all over the world (obviously depending on size) will help a lot. Also allowing them to pay cash on delivery or collection, or by bank transfer, cheque or online methods would give plenty of options. Different people like to be able to pay in different ways.
Give great customer service as well. Make sure that if customers ask you questions that you answer them quickly and honestly. Give them as much information as you can and be polite when you are dealing with them and they will be more likely to buy from you, recommend you and come back and buy again. They will also be happier to pay a higher price for the items, particularly if all of the other tips have been followed as well.