To sell or buy on ebay, you need a Paypal account. I have been buying and selling through ebay for over decade, but not clothes. I have looked at clothes on ebay but have just been overwhelmed by the sheer quantity available, which means that obviously people find it worthwhile. However, you need to take some good pictures of the items you want to sell, preferably with someone wearing them, pictures of clothes lying flat on a bed, makes them look dull. Take pictures from all angles, and details of the garment like any embellishment. Ideally the manufacturers name should be known so people can know the quality to expect and whether it is true to size.
Have a good look at similar garments for sale and consider which ones attract your interest and then do much the same as those sellers.
Alternatively, you could consider selling through a dress exchange. a shop that sells very good quality, perfect condition clothing for a commission.
The thing to do is try to sell one garment see how it goes and proceed on the basis of that experience.