That's up to you! There are pros and cons for using the buy-out feature. If you are unsure of the value of an item, you might consider not using the buy-out feature, and setting the starting bid at the lowest amount you're willing to accept for the item. Or, setting the starting bid at something attractively low, and setting your reserve price for the lowest amount you're willing to accept for the item. That way, people can bid on your item until your auction is over. You might be surprised, someone may be willing to pay more for your item than you would have thought. If you would have set the buy-out price, you may have undersold yourself.
However, the buy-out feature has its perks too. Some people really like the immediacy of the buy-out feature and prefer to buy from auctions where they don't have to wait until the auction is over to win the item. Some buyers also think it's nice to turn over auctions quickly.