For selling items, it's best to keep the item you want to sell in the best condition possible when using it. For choosing the price, I put the original price of items I buy as part of the description on my Possessions Trello board. I then see the prices of other same or similar items on the marketplace and price the item accordingly. Don't price it too low though, as you will get offers to buy it for less after some time anyway. Price it accordingly.