The place you’re selling doesn’t have the right people to buy it. For example you’re trying to sell seriously good quality kit at a car boot and no one understands the value. If you have amazing fashion items sell them where people understand what you have.
Next up consider the price. Unless the goods are totally sold out and current season or you just cannot purchase them and they are highly desirable they have to be some way under the retail price simply because they are second hand. Thats why it is often good to get someone else to sell your items for you as they are not emotionally attached.
The age - if the item is a few seasons old it could be that they are just not the desirable any more: https://laugh-lines.net/wp-content/uploads/2015/03/do-me-sweater.jpg
Product photography and description. If you can’t be bothered to sell it properly why would anyone want to buy it. Step away from your laptop or phone and come back in a few hours. Make sure you look at other images and descriptions of similar products to get your eye in.
You’re selling the right thing at the wrong time. You need to sell items you most want to keep.